ttt8,
I think that the axle vent hollow bolt also secures the brass brake line junction block to the axle tube....
(Might have this confused with another vehicle, but most cars/trucks vented their axles this way.)
Should be a gizmo that looks like an 1/8" pipe nipple with a thing hex on it for a wrench, more nipple, then a little cap crimped on top, loose enough to vent, but tight enough not to come off.
The vent hose deal came along in the '60s.
