I know the chip is quite old as it's been in my possession for quite some time, three or four years easily. It doesn't have any identifying marks and is too old to be DDR2 or anything like that.

I would have to agree that it most obviously resembles SO-DIMM, it doesn't seem to match anything else, either very old or more recent. When I looked at the pin outs on both older and newer units, I do agree that it looks more recent than those chips used in the 386 era. So I agree with your (Syphus) conclusion on that. As to the chip layout, I have no real idea.

I guess we'll never get to the bottom of it anyway, so it'll remain a mystery Thanks for all your replies though, appreciated.