Older A3000s (like mine) had EPROM pinout designed for 27C2048 EPROMs whereas the ROM pinouts for later production runs are for 27C400 EPROMs. ROM tower is just an adapter to make the pinout of newer ROM chips compatible with the rear sockets. You could also burn your own ROM image from standard ROMs to a pair of 27C2048s and would then have a pair of rear socket ROMs.
Mikko.