The 7013-59H planar utilizes two of these 0.5 MB, 12 ns modules as L2 cache, and it seems to be the only model using them at all.

The SRAMs are 5 pieces per module, XCM67B618FN10 made by Motorola Semiconductor. No datasheet available, but looks similar to the MCM67B618, of which the datasheet can be found.
It is a "64K x 18 Bit BurstRAM(TM) Synchronous Fast Static RAM With Burst Counter and Self-Timed Write".
Learning to speak Motorola/NXP/Freescale finds this:

MCM = "Motorola Component Memory"
ENG = Alpha product
XCM = Unqualified standard product
MCM = Qualified standard product
SCM = Qualified standard product with deviations Industrial Temp. part

Now what -IS- an "Unqualified standard product", actually?

Anyone recognizing these modules, maybe from other systems?

One of mine is reported dead at the RS/6000 cache POST (error code 20c), and diagnostics report one being faulty.
If I swap modules, the error moves from slot 1 to 2, so it's indeed a faulty module, not the planar (luckily, this time).
The faulty module looks clean, all passive components are fine, no shorted capacitors, no cold solder joints. All pins of all chips seem to have good connection to the module's edge connector, and all of the PCBs vias and traces measure fine across both sides and ends.

Maybe someone has an idea of what to check, before I order 5 pcs of MCM67B618 as a potential replacement, if they're availabe at all.

For pictures:

http://www.holzapfel.biz/7013-59H/cache.jpg

Here's another picture, compared to a standard 72-pin SIMM:

http://www.holzapfel.biz/7013-59H/cache2.jpg