@antiseptic Anyone?

I believe both the ER605 V1 and V2/V2.6 use the MT7621 based on these steps.

V1:

1. Download the V1 open source code from https://www.tp-link.com/us/support/gpl-code/
2. Extract the archive file
3. Open the README in the root folder
4. Note the reference to "XXX/er605_gpl/tplink/u-boot/mips-2012.03/bin/mipsel-linux-"
5. Based on this, mips-2012.03 should correspond to MT7621 for uboot

V2/V2.6:

1. Download the V2/V2.6 open source code from https://www.tp-link.com/us/support/gpl-code/
2. Extract the archive file
3. Open the product.mk file in the root folder
4. Note the reference to "target/linux/ramips_v2/mt7621/config-4.4.198"

It is interesting to note that V1 appears to use Linux Kernel 3.10.1, but V2 is using 4.4.198. (Based on the kernel target listed in the product.mk file of each open source archive)

Also, all ER605 firmwares seem to be based on OpenWrt Barrier Breaker 14.07. This can be found in the /etc/openwrt_release file when extracting any firmware (.bin) file.