Extroot; Accessing original root overlay

I am trying to follow some instructions here to allow for original root overlay mounting.

At the CLI I see

root@EA3500:~# cat /proc/mtd
dev:    size   erasesize  name
mtd0: 00080000 00004000 "u-boot"
mtd1: 00004000 00004000 "u_env"
mtd2: 00004000 00004000 "s_env"
mtd3: 01400000 00004000 "kernel1"
mtd4: 01170000 00004000 "rootfs1"
mtd5: 01400000 00004000 "kernel2"
mtd6: 01170000 00004000 "ubi"
mtd7: 01600000 00004000 "syscfg"

Which tells me that the original root overlay should be mtd4, so I've modified my fstab as follows

config mount
	option target	/overlay-boot
	option device	/dev/mtd4 (I also tried /dev/mtdblock4)
	option fstype	jffs2
	option options	rw,sync
	option enabled	1
	option enabled_fsck 0

After reboot LUCI shows mount points as follows

(or /dev/mtdblock4 respectively)

I am not sure, if I did something wrong or, if I am missing something conceptually?