Okay, I have tested a whole bunch of memory and here are my findings.
The only SIMMs that work have 12 or 24 chips. Some threw me off because they had a row of 10 on each side and one row of 4 chips in another orientation on one side... but it all works out to 24 chips regardless. Absolutely none of the memory sticks I tried with anything but 12 or 24 chips worked in this system.
Many of the sticks with 12 or 24 chips do not work for some other reason, and the vast majority of them were 16MB SIMMs. I have read that this model should support 64MB of RAM, but none of the 16MB SIMMs I tried worked at all. Most of them immediately gave a 0002151E error, which I can't find anything about specifically. One page says 2151 is "Lost data set ready during data wrap" ... whatever that means. 😮 I have some brand new, sealed pairs of 16MB FP SIMMs I bought for use in AWE32 cards 6-7 years ago, and that also did not work. As far as I can tell, all of the ones that did not work have chips on them that contain "17400" in them.
... oh duh! I guess it probably just can't handle 16MB SIMMs at all. To get 64MB you would just populate both of the RAM card slots, so you'd have 8 SIMMs installed... 8MB each. I only have one RAM card. DERP! That makes more sense. Anyway, this big revelation of mine is probably common knowledge in PS/2 land, but it feels good to have confirmation through testing.
I did find several 4MB sticks that worked. I was very happy to see at least one set of 2x8MB sticks worked, so I should be able to get this system to 28MB at least. I would imagine that Windows 95 runs a tiny bit better with 28MB versus 16MB, especially with a slow old 140MB hard drive. It does irk me just a tiny bit that I won't be able to get it to 32MB since I can't seem to find any other 8MB SIMMs with the correct configuration... but I really doubt it will matter much.
Also, some that worked were 60ns, and some were 70ns.
This about sums up my findings. It was definitely not an issue of bad RAM. That one stick was just incompatible. Several of the ones I installed would give errors (no memory at all), but almost all of the ones that had 8, 9 or 16 chips simply made the system hang at the blinking cursor, which is what I was originally dealing with. I'm guessing someone threw that RAM stick in there right before it was packed away for 15 years. Oh well. It forced me into my first PS/2 learning experience. 😜
EDIT: Whelp... it doesn't like 28MB (8+8+8+4). It works with 24MB (8+8+8) but if I add another 4MB stick it only detects 12MB... or sometimes it will hang before the memory check, even those all of those sticks work individually. It may be possible to find the right combination but it's proving to be a bit of a pain.
... so, 24MB it is for the 'ol PS/2!
EDIT2: Okay... what gives? Windows 95 is reporting 16MB of RAM even though the BIOS detects 24MB. I am using 8+8+4+4 SIMMs.
Is this a PS/2 issue or a Windows 95 issue? I have, admittedly, not dabbled with 95 much over the years.
Now for some blitting from the back buffer.