Okay, here's my report. Also, I added 3 photos of the derailer, mounted on the bike, to my initial post at the top.
Installation: the lower pulley wheel turns with some resistance, and remains stiff after pulling out the (steel) bushing and applying some thick oil. But it spins okay as-is, and will free up with time. Putting the derailer on the bike and adjusting it was no problem, same as any indexed rear derailer. In the stand, the derailer shifted fine, with no need to over-shift. I'm running a 9-speed drivetrain with Sram 12-26 cassette, Shimano HG-90 chain, and Shimano 105 STI.
Riding performance: I went on a 25-mile ride today and am very pleased with the shifting. Crisp, quick and precise, as I would expect running a 10-speed rear derailer on a 9-speed drivetrain. Much cleaner shifting than the XT long-cage, 8-speed era rear derailer that was on there previously, and with no change in any other factors. I was wondering if the cable housing by the rear derailer (which was old used housing prior to my putting it on this bike) was a cause of less-than-perfect shifting, but apparently everything comes down to the rear derailer.
The old XT M737 rear derailer will serve some 8-speed touring or mountain bike well in the future.
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"c" is not a unit that measures tire width