Sorry to come back on this. But after a night's sleep I think I have a better idea.

Both names have been in use for years for the same file.
"BasiliskII_keycodes" with BasiliskII and "keycodes" with SheepShaver is true for the emulators distributed for OSX/macOS, but I am not sure which files are used on other platforms or by users who compile/build themselves.

Maybe make it like this for both BasiliskII and SheepShaver:

If "keycodes true" and no keycodefile or path defined
Look for "BasiliskII_keycodes"
If no "BasiliskII_keycodes"
Look for "keycodes"

I think it is better that BII/SS try to read first is not BasiliskII_keycodes but keycodes.
If you put all of SheepShaver, keycodes and BasiliskII_keycodes in the same folder, I think it is more natural to give priority to keycodes.

The average user will not notice the difference, but I agree that it feels more natural to give "keycodes" priority. If ever other file names will circulate (I hope not) those could be added easily.

Edit: Judging by the comments at the top of the file, "keycodes" even was the original file name.

Changed.