It's reminiscent of ARM, but the relevant part is that the CAS instruction's second word bits 5:0 look like a "modrm" (to use the x86 terminology) where the officially documented values select only Dn, but the undocumented variant would correspond to (d16,An). At least, that's my theory for why A1 gets modified.
http://goldencrystal.free.fr/M68kOpcodes-v2.3.pdf
It's reminiscent of ARM, but the relevant part is that the CAS instruction's second word bits 5:0 look like a "modrm" (to use the x86 terminology) where the officially documented values select only Dn, but the undocumented variant would correspond to (d16,An). At least, that's my theory for why A1 gets modified.