Yes, the APTR / BPTR divergence had something to do with AmigaDOS (the disk operating subsystem) was originally written with BCPL. While it was later rewritten

Yes, the APTR / BPTR divergence had something to do with AmigaDOS (the disk operating subsystem) was originally written with BCPL. While it was later rewritten in C, however, the ABI had to remain compatible and thus the pointer-mangling continued. I don't remember if they had any use for the extra two bits or whether the addressing scheme was coined particularly just to access a 4x large address space.