From a bit of googling, this seems to be what it is. I thought I'd try to confirm by looking up what the sizes of the spindles that it would go on are. You'd think that sort of thing would be easy to find...
I've found a site that said that the sizes had been standardised in 1972 to one or other of two sizes, but my google foo has been completely unable to figure out what those two sizes actually are!
This one has two tapered square drive sockets, one tapering from about 32 mm to 30 mm and one tapering from 26 mm to 22 mm.