Scott-Taggert ST-22
DC Audio Amplifier Triode
1926 (British)

S.T. Ltd. was formed by British tube pioneer John Scott-Taggert in 1926. The tubes were actually made by Mullard for Scott-Taggert. The ST-22 was the same as the Mullard PM1LF. The S.T. company seems to have not lasted past 1927.