I am in agreement with the posts here, that looks very home made. Or at least built one at a time. There's half a dozen details that just would not lend themselves to batch production.
Assuming it's in good shape, it might be quite serviceable though, and if so, could be a quite valuable addition to a tractor that's used more than it's displayed.
I don't see much in the way of chain guides or tensioning adjustments. It may be a little loud. Probably not obnoxiously so, or maybe that extra thing I can't identify on the back of the chain is it and I just can't see it. Really cool if it's there, but if not, no biggie. I'd also count the teeth on the two gears, that'll let you figure the output speed. Only from the pictures, It looks to me like it's slowed down too much to be the "factory speed", and geared up too much to be a standard 540 rpm. Again, probably quite a useful piece, but that's a piece of trivia that you really want to know before you over torque the transaxle mounts with aggressive use of an implement at very high gear reduction, or before you ride a mower into low earth orbit with not enough gear reduction.
I've seen pictures only of one other home made PTO, and I really like the simplicity of this one. Like others have said, this might just turn out to be a really useful asset.
