About 2 hours ago I bought the R6620, and have not yet flashed it. I'm no expert, but perhaps the following OEM information will help someone else in this thread. I have also dumped devices 0 to 20, download can be found here
# cat /proc/mtd
dev: size erasesize name
mtd0: 00100000 00020000 "Bootloader"
mtd1: 00100000 00020000 "SC PID"
mtd2: 00400000 00020000 "Kernel"
mtd3: 01c00000 00020000 "Rootfs"
mtd4: 00200000 00020000 "English UI"
mtd5: 00200000 00020000 "ML1"
mtd6: 00200000 00020000 "ML2"
mtd7: 00200000 00020000 "ML3"
mtd8: 00200000 00020000 "ML4"
mtd9: 00200000 00020000 "ML5"
mtd10: 00100000 00020000 "Factory"
mtd11: 00100000 00020000 "SC Private Data"
mtd12: 00200000 00020000 "POT"
mtd13: 00200000 00020000 "Traffic Meter"
mtd14: 00200000 00020000 "DPF"
mtd15: 00200000 00020000 "SC Nvram"
mtd16: 00200000 00020000 "Ralink Nvram"
mtd17: 00200000 00020000 "Ralink Reserved"
mtd18: 00200000 00020000 "ML6"
mtd19: 00200000 00020000 "Upgrade Flag"
mtd20: 00200000 00020000 "Reserved Block3"
mtd21: 03c00000 00020000 "Reserved Block4"
# mtdinfo -a
Count of MTD devices: 22
Present MTD devices: mtd0, mtd1, mtd2, mtd3, mtd4, mtd5, mtd6, mtd7, mtd8, mtd9, mtd10, mtd11, mtd12, mtd13, mtd14, mtd15, mtd16, mtd17, mtd18, mtd19, mtd20, mtd21
Sysfs interface supported: yes
mtd0
Name: Bootloader
Type: nand
Eraseblock size: 131072 bytes, 128.0 KiB
Amount of eraseblocks: 8 (1048576 bytes, 1024.0 KiB)
Minimum input/output unit size: 2048 bytes
Sub-page size: 2048 bytes
OOB size: 64 bytes
Character device major/minor: 90:0
Bad blocks are allowed: true
Device is writable: true
mtd1
Name: SC PID
Type: nand
Eraseblock size: 131072 bytes, 128.0 KiB
Amount of eraseblocks: 8 (1048576 bytes, 1024.0 KiB)
Minimum input/output unit size: 2048 bytes
Sub-page size: 2048 bytes
OOB size: 64 bytes
Character device major/minor: 90:2
Bad blocks are allowed: true
Device is writable: true
mtd2
Name: Kernel
Type: nand
Eraseblock size: 131072 bytes, 128.0 KiB
Amount of eraseblocks: 32 (4194304 bytes, 4.0 MiB)
Minimum input/output unit size: 2048 bytes
Sub-page size: 2048 bytes
OOB size: 64 bytes
Character device major/minor: 90:4
Bad blocks are allowed: true
Device is writable: true
mtd3
Name: Rootfs
Type: nand
Eraseblock size: 131072 bytes, 128.0 KiB
Amount of eraseblocks: 224 (29360128 bytes, 28.0 MiB)
Minimum input/output unit size: 2048 bytes
Sub-page size: 2048 bytes
OOB size: 64 bytes
Character device major/minor: 90:6
Bad blocks are allowed: true
Device is writable: true
mtd4
Name: English UI
Type: nand
Eraseblock size: 131072 bytes, 128.0 KiB
Amount of eraseblocks: 16 (2097152 bytes, 2.0 MiB)
Minimum input/output unit size: 2048 bytes
Sub-page size: 2048 bytes
OOB size: 64 bytes
Character device major/minor: 90:8
Bad blocks are allowed: true
Device is writable: true
mtd5
Name: ML1
Type: nand
Eraseblock size: 131072 bytes, 128.0 KiB
Amount of eraseblocks: 16 (2097152 bytes, 2.0 MiB)
Minimum input/output unit size: 2048 bytes
Sub-page size: 2048 bytes
OOB size: 64 bytes
Character device major/minor: 90:10
Bad blocks are allowed: true
Device is writable: true
mtd6
Name: ML2
Type: nand
Eraseblock size: 131072 bytes, 128.0 KiB
Amount of eraseblocks: 16 (2097152 bytes, 2.0 MiB)
Minimum input/output unit size: 2048 bytes
Sub-page size: 2048 bytes
OOB size: 64 bytes
Character device major/minor: 90:12
Bad blocks are allowed: true
Device is writable: true
mtd7
Name: ML3
Type: nand
Eraseblock size: 131072 bytes, 128.0 KiB
Amount of eraseblocks: 16 (2097152 bytes, 2.0 MiB)
Minimum input/output unit size: 2048 bytes
Sub-page size: 2048 bytes
OOB size: 64 bytes
Character device major/minor: 90:14
Bad blocks are allowed: true
Device is writable: true
mtd8
Name: ML4
Type: nand
Eraseblock size: 131072 bytes, 128.0 KiB
Amount of eraseblocks: 16 (2097152 bytes, 2.0 MiB)
Minimum input/output unit size: 2048 bytes
Sub-page size: 2048 bytes
OOB size: 64 bytes
Character device major/minor: 90:16
Bad blocks are allowed: true
Device is writable: true
mtd9
Name: ML5
Type: nand
Eraseblock size: 131072 bytes, 128.0 KiB
Amount of eraseblocks: 16 (2097152 bytes, 2.0 MiB)
Minimum input/output unit size: 2048 bytes
Sub-page size: 2048 bytes
OOB size: 64 bytes
Character device major/minor: 90:18
Bad blocks are allowed: true
Device is writable: true
mtd10
Name: Factory
Type: nand
Eraseblock size: 131072 bytes, 128.0 KiB
Amount of eraseblocks: 8 (1048576 bytes, 1024.0 KiB)
Minimum input/output unit size: 2048 bytes
Sub-page size: 2048 bytes
OOB size: 64 bytes
Character device major/minor: 90:20
Bad blocks are allowed: true
Device is writable: true
mtd11
Name: SC Private Data
Type: nand
Eraseblock size: 131072 bytes, 128.0 KiB
Amount of eraseblocks: 8 (1048576 bytes, 1024.0 KiB)
Minimum input/output unit size: 2048 bytes
Sub-page size: 2048 bytes
OOB size: 64 bytes
Character device major/minor: 90:22
Bad blocks are allowed: true
Device is writable: true
mtd12
Name: POT
Type: nand
Eraseblock size: 131072 bytes, 128.0 KiB
Amount of eraseblocks: 16 (2097152 bytes, 2.0 MiB)
Minimum input/output unit size: 2048 bytes
Sub-page size: 2048 bytes
OOB size: 64 bytes
Character device major/minor: 90:24
Bad blocks are allowed: true
Device is writable: true
mtd13
Name: Traffic Meter
Type: nand
Eraseblock size: 131072 bytes, 128.0 KiB
Amount of eraseblocks: 16 (2097152 bytes, 2.0 MiB)
Minimum input/output unit size: 2048 bytes
Sub-page size: 2048 bytes
OOB size: 64 bytes
Character device major/minor: 90:26
Bad blocks are allowed: true
Device is writable: true
mtd14
Name: DPF
Type: nand
Eraseblock size: 131072 bytes, 128.0 KiB
Amount of eraseblocks: 16 (2097152 bytes, 2.0 MiB)
Minimum input/output unit size: 2048 bytes
Sub-page size: 2048 bytes
OOB size: 64 bytes
Character device major/minor: 90:28
Bad blocks are allowed: true
Device is writable: true
mtd15
Name: SC Nvram
Type: nand
Eraseblock size: 131072 bytes, 128.0 KiB
Amount of eraseblocks: 16 (2097152 bytes, 2.0 MiB)
Minimum input/output unit size: 2048 bytes
Sub-page size: 2048 bytes
OOB size: 64 bytes
Character device major/minor: 90:30
Bad blocks are allowed: true
Device is writable: true
mtd16
Name: Ralink Nvram
Type: nand
Eraseblock size: 131072 bytes, 128.0 KiB
Amount of eraseblocks: 16 (2097152 bytes, 2.0 MiB)
Minimum input/output unit size: 2048 bytes
Sub-page size: 2048 bytes
OOB size: 64 bytes
Character device major/minor: 90:32
Bad blocks are allowed: true
Device is writable: true
mtd17
Name: Ralink Reserved
Type: nand
Eraseblock size: 131072 bytes, 128.0 KiB
Amount of eraseblocks: 16 (2097152 bytes, 2.0 MiB)
Minimum input/output unit size: 2048 bytes
Sub-page size: 2048 bytes
OOB size: 64 bytes
Character device major/minor: 90:34
Bad blocks are allowed: true
Device is writable: true
mtd18
Name: ML6
Type: nand
Eraseblock size: 131072 bytes, 128.0 KiB
Amount of eraseblocks: 16 (2097152 bytes, 2.0 MiB)
Minimum input/output unit size: 2048 bytes
Sub-page size: 2048 bytes
OOB size: 64 bytes
Character device major/minor: 90:36
Bad blocks are allowed: true
Device is writable: true
mtd19
Name: Upgrade Flag
Type: nand
Eraseblock size: 131072 bytes, 128.0 KiB
Amount of eraseblocks: 16 (2097152 bytes, 2.0 MiB)
Minimum input/output unit size: 2048 bytes
Sub-page size: 2048 bytes
OOB size: 64 bytes
Character device major/minor: 90:38
Bad blocks are allowed: true
Device is writable: true
mtd20
Name: Reserved Block3
Type: nand
Eraseblock size: 131072 bytes, 128.0 KiB
Amount of eraseblocks: 16 (2097152 bytes, 2.0 MiB)
Minimum input/output unit size: 2048 bytes
Sub-page size: 2048 bytes
OOB size: 64 bytes
Character device major/minor: 90:40
Bad blocks are allowed: true
Device is writable: true
mtd21
Name: Reserved Block4
Type: nand
Eraseblock size: 131072 bytes, 128.0 KiB
Amount of eraseblocks: 480 (62914560 bytes, 60.0 MiB)
Minimum input/output unit size: 2048 bytes
Sub-page size: 2048 bytes
OOB size: 64 bytes
Character device major/minor: 90:42
Bad blocks are allowed: true
Device is writable: true