The US image is 260 kB smaller than the EU image, so the device might have different partitioning (didn't check).
The firmware had soft_ver 1.0.1 (compared to 1.0.0 for the EU version), so I changed to 3.0.1 (instead of 3.0.0 for the EU version). This might become a problem when flashing OpenWrt again.
It worked, I am back on the stock firmware! That's genius, thank you so much!!!
Some notes:
maybe part of the size difference is due to the languages that are supported;
i'm able to go to back and forth between openwrt and the stock firmware;
by reading the source code of openwrt, I was able to understand how to fix the chksum. I applied the same fix to the CA version of the firmware hoping to get the North American radio regulations with a web UI in French. In the end the French is not available but I'm now running a more current version 3.1.0:
Update: WARNING: Actually after some tests it looks like I'm not able to install openwrt after having upgraded to the CA version. *** Use at your own risk ***