I uploaded a (crappy - sigh) image of an H215 I have to here: https://gunkies.org/wiki/File:H215-core-memory-board.jpg and it's clearly not symmetrical, but does have a slightly bugger blank space than yours. It has a label that says "18x8K" (so 16 bits plus byte parity - for DATOB bus write operations to one byte at a time), which sounds orrect. No, this way pre-dates ECC memory (the cost of the extra bits - more cores to string in - was probably more than it was worth), and probably before ECC disks too (I think the first DEC ECC disk was the RP04). That's my guess too; _but_ there might be some jumpering/etc issues to adjust the word width. I.e. the PDP-11 parity H215 is 18 bits wide, which is right for 16-bit words, whereas PDP-10 parity memory would be 36+1 parity bit = 37 bits wide. So there might be 2 banks on one of those cards, and the PDP-11 memory treats them as separate banks, whereas on a -10 they'd be ganged together in parallel. (Whether that's all done on the companion driver boards, and the H21x card would just bring the wiring of the two banks out to the edge connector in parallel, letting the driver board do what it wants, I don't know - you'd have to look at the MM11-L engineering drawings/TM.) The other possibility is that the PDP-10 memory used these boards in pairs. We do have the MA10 Maint Manual, but in a quick look it looks like it doesn't use these boards. So maybe a later -10 memory system? Noel