as I said ,it is a old style twist on breather, the hole way to large for a grommet & a regular push in pvc. used on newer style valve covers.
the valve covers are cal-custom finned covers, mid-60's.
the breathers fit into the valve cover hole & twist on & lock on the valve covers, the same way a radiator cap locks onto the radiator.
I don't think there is a issue of any kind of pressure going from the manifold to the valve cover,(engine is not backfireing thur the carb). it only make the popping noise, when the engine is up to running temp, been driven, & is then idleing, like stoped at a light.
I am going to re install the breather/pvc on the other side of the engine to see if it starts popping on the left side, will post