I'm not a metalurgist, just a weight weenie mountain biker.
From work, most of the cast stuff I run into seems to have a much lower melting point than extruded material does I see this in poles with cast bases when we need to heat the anchor nuts to remove them after they sieze. Which could explain the joining of the pliers and tail piece.
Would that air entrapment make it more favourable sonically? It would make it lighter than extruded for sure.
I would guess that in the 50's that they would be cast is likely. In the grand scheme of things, it would still be a rather small production.
I'm just thinking out loud, but I would have guessed that with the bridges and tail pieces, it would have been pretty easy to have them cast at a local foundry for cheap back in the day.
(Motor city being up the road may have made it pretty easy to get parts cast and plated pretty cheaply.)
Finishing that thought, these days, most of your aluminium parts for a small run would be made in a machine shop, rather than a foundry, does that make it more likely to have an alloy with more magnetic properties? Or that is more resonant?