This is the closest I could find to what the 68000 opcode map looks like:

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.