For $125, I figure it would need to have hammer divots in it, that you could see over the land-line phone, to not be a decent buy!
Not certain, but I would not be surprised if it was OEM equipment on a tool and cutter grinder, or maybe a small benchtop mill.
Be nice to know the center height, but at the price, if I didn't have a couple other indexing methods already to hand, I'd just grab it. Even more so if I was close!
New plates are pretty easy to make. Just gotta remember, that any error in the position of the hole in the plate, is reduced by the ratio of the worm and wheel!
FWIW, I made a couple indexing plates during a lunchtime at work one day. I used gears that were with the lathe we had, built an adapter-spindle to link the plate solidly to the gear, and, made a spring loaded indexing mechanism (literally a chunk of flat stock pulled from the scrap bin, and a pair of vice-Grips), and pounded out a 127, 100, and 63 hole set of circles on some plate stock on a drill press. Done, before lunch was!