I agree. Some later A500s had a capacity for more chip ram that was later added, and marketed as A500+.
I had an early A500 (1988 but I don't know the rev) that had a trapdoor expansion and there was a hack we did back then where you could cut a trace on the motherboard and get the A500 to configure the trapdoor fast RAM as CHIP ram.
On that basis I'd say the trapdoor ram was in the motherboard RAM address space.
