Crane Post-WW2 Tub Shower escutcheon, early style.
This is the earlier version of the Crane tub/shower escutcheon, and was commonly used in the 1950’s. It is distiguished by the hex-shaped sides to allow one to use a wrench on it to tighten or loosen. These are solid brass with a nice plate of chrome on them. OD is 3″, and they stand a total of 1 1/2″ tall.