General Electronics Inc. CDR 3B28 Rectifier, (866A replacement).
NOS General Electronics 3B28 Half Wave Gas Rectifier tube. Has UX 4 pin Base Made in the USA. Tests: 42 (min=40)
This tube appears to be unused, except for some black and white marks on the base between the pins. There's no evidence of any use on the pins or the plate cap.
The tube was tested on a TV7 tester, with a reading of 42, minimum reading being 40.