PDP-11 M873 ROM card info?

[Noel Chiappa]

Does anyone know of _any_ information about this series of cards? They are
quad cards which seem to use two chips of the same types as the M9301 uses
four, with 128 words of memory. They thus must fit between the M792 diode ROM
card, and the M9301, in timing terms. However, search as I can, I don't seem
to be able to find anything on them. Known variants are the -YA, -YB and -YJ.

   Noel