Put the tube in the freezer for a few minutes and when you take it out check on the side for the tube designation. You only have a few seconds when the numbers will be visible.
That’s a 1952 RCA 5U4G. A 5Y3 would be smaller and one from that era wouldn’t have the coke-bottle glass. An RCA 5R4 from that era would probably have a brown base and would look different. A 5V4 would be smaller and would have a different internal structure. A 5Z3 has four pins instead of the octal base.
Put the tube in the freezer for a few minutes and when you take it out check on the side for the tube designation. You only have a few seconds when the numbers will be visible.