I found the BOM and the exact pn for the mk2a encoder. It's an Alps EC11E-15244.

Mouser.com sell this for $5.61, one-off (and they're even in stock!). Changing this part is easier than doing the Mk2 encoder because it's just plain through hole.

Hugo
ps: Now I've actually found the BOM, if anyone else has any exact pn requirements, let me know