PDP-8/A 128K memory - M8417 or M8418?

[Jack Rubin]

I've been looking for a 128K MOS memory board for my PDP-8/A for a while. I finally got one,  but it turned out to be an M8418.

The docs I've seen (bitsavers EK-MS8CD-TM-001, 1980 + printsets) talk about an M8417 with 4k DRAM chips (MS8-C) and an M8417 with 16k DRAM chips (MS8-D), but apparently at some point the 16k versions became known as the M8418. The card I received has 96 Fujitsu MB8116E 16k DRAMs, arranged in two 8 x 12 chip arrays. The actual circuit board says PDP MOS MEMORY - M8417 - 50 12701B on the back but the metal card ejector edge is stamped M8418 JC. Chip dates on the board put it at 1980 production.

None of my literature has the M8418 p/n but most of it may be too old. The M8418 part number (with JC suffix to indicate Fujitsu RAM) does show up in the 1988 Options Module List.

I know several list members have similar boards. How are they marked? Can anyone point to more info on the M8418?

Thanks,
Jack