[mikethedruid]

I have am RCA Radiola Senior, which was made by Westinghouse for RCA and is virtually the same receiver. I looked at my own binding posts and measured them. They are indeed 3/8" diameter, but look to be a fine thread, probably 28 or 32. You can get 3/8" 28 nuts and 3/8" 32 nuts to try, and see which you could use, and you can get taps once you know the right size in case you want to try to make replacement caps from brass pipe of proper diameter, or have a machine shop make some up for you. You may be able to use bicycle axle nuts in 3/8" 28 if that is the right thread pitch.