Quick note for similar later problems: my ancient Optiplex 7010 can run RAM DIMMs with either 1.5v or 1.35v, but it cannot run them at the same time.
I found this out because I bought used DIMMs on Polish ebay equivalent Allegro.pl which worked, and then bought another two DIMMs from the exact same offer (I just used the “buy again” button) which turned out to be different but also worked.
Just they did not work with one another.
It's likely the seller is just stripping RAM from whatever machines they can find and selling them, and their offer leaves details as to voltage out.
So, I will have to get another 2 bars. Hardly a problem but still annoying.