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As for the "second layer of vernier under the turntable", do you mean that really dark circle on the wooden top, right underneath the turntable, and slightly larger ?
If so, I can't say from the photos exactly what this is, but most external-horn Columbias have a nickel-plated dust-ring that surrounds the turn-table, concealing the raw edge of the TT.
Perhaps the dark ring is the remnants of some sort of shim or gasket bewteen the dust-ring and top deck of cabinet ?
As for how to clean rusted metal, instead of sand blasting, consider soda-blasting: this is supposed to be very effective, yet gentle on the metal.
Good luck with this machine - please keep us posted and share photos !
Regards,
De Soto Frank