No, not tube, but one of the first transistorized analog computers. But since it comes from that switchover period, it still uses the older +/- 100v standard that was typical up until that general time. It was right around then when the first +/- 10v machines began to appear, but this certainly isn't one of those. As for it looking like a modular, that's because these were the sort of things the modular synth was modeled after; Don Buchla certainly had experience with them as a researcher with NASA, and I suspect Bob Moog saw a few while in his academic studies, pre-synth.
As for the Comdyna GP-6, that's really about the last analog computer that was being made, right up to around 2000 or thereabouts. Comdyna had another machine that they'd specially specced out for music work, though, although I'm not sure if any of them sold. I forget the model number of that one, tho. After they went out of business (so it seems), I don't think anyone is currently making analog computers anymore. And that's kind of...irritating, actually, because they do have more uses than in just an academic/research setting, but one has to actually know how to program one to get any use out of it and that's a bit of a lost art, it seems. In theory, though, something like one might be cobbled together from discrete synth modules, but the thing that makes the Systron-Donner useful is that it's patched more like a Serge, with stackable banana plugs, which opens some not-exactly-computational behavior up for (ab)use.