[mikethedruid]

ST is not actually a standard abbreviation. I believe I saw it on only one tube I ordered which was Russian made. G is what you call "shoulder type, GT is the later, smaller, straight side version, and later many GT types are marked GT/G to save confusion, clearly showing it replaces either. Except for looks they are the same , BUT, if there is NO SUFFIX, and the tube has a metal outside, it can be a problem in Philco radios. These metal tubes use the case as an RF shield, and it connects to a pin that is connected to the ground plane of the radio. Philco radios use the G or GT types which do not use this pin, and so use the socket solder lug as a tie point, which can be live, so you can't use a metal tube in Philcos safely, metal does not substitute for a G or GT type. This is why many RCA radios don't use tube shields for their RF, oscillator, and IF tubes, the metal tube IS the shield, but Philcos and other radios use those shields and G or GT tubes.