RMerlin
Asuswrt-Merlin dev
As for any minor differences, that would have tobe in the firmware as the hardware is identical.
in the firmware, you can freely use the ASUS firmware for either model on either one - thus turning a "R" into a "U" or "U" into an "R"
The firmware is 100% identical in both cases, it won't turn your router into one or another SKU. The ID is stored in just a string stored in the bootloader, which gets shown whenever the webui wants to display the model number.