No this is a single bolt and thick washer, screws into the end of the shaft. Probably has no use on the manual hubs since the hub body is much shallower.

Nope. That is somebody's aftermarket addition. Most likely they a) did not know what they were doing or b) the splines were shot or c) they saw a threaded hole and were determined that *something* needed to fill it.

I've see it on rigs with destroyed splines. It works, not ideal, but it works.

Michael