That is actually an incredibly intriguing doodad. I would love to know what the contents of that EPROM are, but... if I had to take a wild guess I think what this thing might be for is to dynamically apply a few single-byte patches to the Mac ROM. (If you really feel like a project you could examine the traces and reconstruct the schematic so we could figure out under what circumstances the EPROM is enabled.) Since the Mac uses a 16 bit data path to ROM I think it's pretty unlikely that that there is a complete driver/extension in it.
If I had to hazard a guess, my gut feeling is this device implements a few hacks (which literally might just be changing a few one byte constants) to the hardware initialization routines in order to enable a really old Mac (perhaps only Macs with the original 64k ROMs, which would mesh with the 32k size of the EPROM) to recognize more than 512k or RAM, and thus work with one of their "MacSnap" one or two MB clip-on memory boards. It appears that when said board was sold with a SCSI expansion (with resided in *both* ROM sockets and required 128k ROMs) the complete package was called the "MacSnap 524S". This just says "524", so if the "S" means SCSI...
If I had to hazard a guess, my gut feeling is this device implements a few hacks (which literally might just be changing a few one byte constants) to the hardware initialization routines in order to enable a really old Mac (perhaps only Macs with the original 64k ROMs, which would mesh with the 32k size of the EPROM) to recognize more than 512k or RAM, and thus work with one of their "MacSnap" one or two MB clip-on memory boards. It appears that when said board was sold with a SCSI expansion (with resided in *both* ROM sockets and required 128k ROMs) the complete package was called the "MacSnap 524S". This just says "524", so if the "S" means SCSI...