USA code:

Codes consist of digits, letters, digits, possibly further letters

First digits give heater _voltage_ rating, with the exception that the codes 7 and 14 are used to indicate 6.3V and 12.6V valves with a locatal base.

Letters specify the type of valve, but there appears to be no consitency of coding. S often indicates a single-ended (no top cap) version of an earlier valve with such a cap

Second digits give either the number of active electrodes, or the number of external connections.

Final letters often specify the type of envelope. The code is
G - large glass envelope.
GC - Glass Compact - a tubular glass envelope
GT - Glass Tubular - the smallest glass envelope
M - Metal Envelope
WA - High Quality version

VT numbers are USA miliary valves. Unfortunately no way to decode.