The equivalent bush in a Commando
has been known to migrate, and gets ground away by the next gear on the mainshaft.
I believe they have 2 separate bushes.
I think Triumphs just have one long bush, on pre-units it projects through the inner chaincase sliding seal.
I think you're right that in the units the seal runs on the gear.
When I look closely at the 1st pic, the bush looks far from new, I can only suppose that PO began removing it and then gave up.
God only knows the story. Perhaps he thought it was worn.
I don't think it is possible for the bush to migrate in a living gearbox to that length, so its something that's happened through attempted maontenance.
You've pressed it back in successfuly. Doing that on a long, slender bush like this tends to squash it up a little (both OD and ID) so it should stay where it is, as long as the shaft still goes through freely.