On the plus cutting that one resistor (not sure off hand which one) allows 4 Meg's of ram to be seen and used. That may be the case with the 512 upgrade. The only difference is the ram chips being larger in capacity and that jumper area to my knowledge, that's why yours is interesting.

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I don't follow. The Plus has either resistors (early) or 3 prong jumper array (later). But in the case of this 128k board (630-0101) its certainly stock less the LED, resistor at R11 and the connector where normal 512k upgrades have a board or such soldered. With this board, that upgrade plugs in instead with a cable. Thats a little weird, but the real oddity is another cable with a plug on the end that seems to go nowhere.

As for the hybrid board produced for the 512k, with the option of also being a 128k board, there are some pretty noticeable differences compared to the original 128k board.

What I am thinking is maybe this board can be enabled or disabled with a switch? It certainly seems like a possibility as I see no other reason for the strange cable.