That's a 512k expansion board, something that just about every A500 in the world has.. This is one of the (physically) smaller sized ones, using large chips to get 512k..
There are many many different models of ram expansions for the A500. Just about everyone made them, since the design is so simple.. Just connect the chips to the bus and enable exram.
KCS made a PC-emulator for the trapdoor too, but mainly it's for ram expansions only on the A500. (no turbo cards like with the A1200)
It probably doesn't make a difference to the games, most need 1MB anyway. The switch is for switching the expansion on and off.
