Well now I feel stupid:
root@ER605:/# df -h
Filesystem Size Used Available Use% Mounted on
rootfs 61.3M 852.0K 60.5M 1% /
/dev/root 11.6M 11.6M 0 100% /rom
tmpfs 61.3M 3.1M 58.2M 5% /tmp
root 61.3M 852.0K 60.5M 1% /tmp/root
overlayfs:/tmp/root 61.3M 852.0K 60.5M 1% /
/dev/mtdblock12 1.5M 652.0K 884.0K 42% /tmp/userconfig
tmpfs 512.0K 0 512.0K 0% /dev
tmpfs 61.3M 3.1M 58.2M 5% /tmp/run/netns
That looks like 16mb Flash on the TL-R605/ ER605 v1, and then ran in RAM as 64mb overlay
root@ER605:~# uname -a
Linux ER605 3.10.108 #19 SMP Sat Oct 15 09:36:30 CST 2022 mips GNU/Linux
root@ER605:~# cat /proc/mtd
dev: size erasesize name
mtd0: 00040000 00010000 "bootloader"
mtd1: 00030000 00010000 "bootloader-fs"
mtd2: 00010000 00010000 "extra-para"
mtd3: 00180000 00010000 "kernel"
mtd4: 00bf0000 00010000 "rootfs"
mtd5: 00020000 00010000 "panic-oops"
mtd6: 00010000 00010000 "partition-table"
mtd7: 00010000 00010000 "device-info"
mtd8: 00010000 00010000 "support-list"
mtd9: 00010000 00010000 "firmware-info"
mtd10: 00010000 00010000 "tddp"
mtd11: 00020000 00010000 "log"
mtd12: 00180000 00010000 "rootfs_data"
mtd13: 01000000 00010000 "firmware"
[ 1.700000] GD25Q128C(c8 40180000) (16384 Kbytes)
[ 1.704000] mtd .name = raspi, .size = 0x01000000 (16M) .erasesize = 0x00010000 (64K) .numeraseregions = 0
[ 1.712000] @@-----debug tplink flash parse--------
[ 1.720000] 14 tp-link partitions found on MTD device raspi
[ 1.724000] Creating 14 MTD partitions on "raspi":
[ 1.728000] 0x000000000000-0x000000040000 : "bootloader"
[ 1.736000] 0x000000040000-0x000000070000 : "bootloader-fs"
[ 1.740000] 0x000000070000-0x000000080000 : "extra-para"
[ 1.748000] 0x000000080000-0x000000200000 : "kernel"
[ 1.752000] 0x000000200000-0x000000df0000 : "rootfs"
[ 1.760000] mtd: device 4 (rootfs) set to be root filesystem
[ 1.764000] 0x000000df0000-0x000000e10000 : "panic-oops"
[ 1.772000] 0x000000e10000-0x000000e20000 : "partition-table"
[ 1.780000] 0x000000e20000-0x000000e30000 : "device-info"
[ 1.784000] 0x000000e30000-0x000000e40000 : "support-list"
[ 1.792000] 0x000000e40000-0x000000e50000 : "firmware-info"
[ 1.796000] 0x000000e50000-0x000000e60000 : "tddp"
[ 1.804000] 0x000000e60000-0x000000e80000 : "log"
[ 1.808000] 0x000000e80000-0x000001000000 : "rootfs_data"
[ 1.816000] 0x000000000000-0x000001000000 : "firmware"