Mystery re: MCM66128L20

I am looking for information about this part; I have something of a mystery on my hands. At issue is a 512K IBM memory card that is part of the PC/XT 370 option (from 1984). The card I have has 18 stacked chips comprising 512K of RAM; the top chip of one of the pairs was broken off. The top chips are stamped MCM66128L20; the bottom chips have no part number. I could only find the part through a broker (and I paid $25 for it). I can't find a spec sheet but expected to receive a stacked part (most likely, two 64K chips stacked to make a "128K" chip. When the new chips arrived (after I signed munitions documents), they weren't stacked pairs. That left me to wonder whether the new parts are a later rev that incorporates all the circuits into one package or whether (more troublesome) the bottom chip of the pair is now a complete unmarked mystery. The chips were soldered together, not welded. So . . . I soldered one of the new chips onto the top of the bottom chip. I figured that if that action was a bust, I could still remove the bottom chip and replace both with one of my new ones. Much to my surprise, the board now passes memory diagnostics. Does anyone have a book with a spec sheet on this part number and possibly any insight into what is going on with the (unmarked) bottom one? There are some pictures of the project at:- www.xt370.net including the board and the chips. On the second page (click on 'more restoration photos') you can see the missing top chip. The code on the new chips is 8407BQD. The old chips say BQD8431. Thanks, Michael Ryan