This black plate Super Silvertone 5Y3G rectifier tube was tested on our Paul Hart-calibrated TV7/U for shorts, gas and transconductance. There were no shorts or gas.
It measured 57/60 where minimum (65%) is 40/40 and 100% is 62/62.
It has a nifty, old-fashioned Coke bottle; a D getter, good flashing for its type and a 41X date code.