The 120 should work with either cable, AT or PS2, black or beige. I have never had a 120 but I have had a couple of 131s and I think they are the same.
I am pretty sure that the originals came with a "short" black coiled cable with the large 180 degree DIN connector. That one should work with a DIN-to-PS2 adapter.
Those would be less desirable for someone actually using the board, but I understand that you want the old one for collecting. I might have 2 of those, if I find the extra I would consider selling it to you.