Using the correct? firmware image, my AR-300m-ext says 'wrong hardware'?

Hello,
I am trying to install the pre-release image ‘firmware/ar300m/nand/testing/gl-ar300m-3.013-0128.img’ on my AR-300M-ext.

Download seems to be ok, upload using the web interface was stated as ‘sucessful’, but then the upgrade exits with ‘wong hardware’.

Any hints what went wrong?

greetings,
Raeuberguard

download the tar file