The SH342 is a thermal delay tube. The maker is ITT but the device was made in the UK. So in fact this was made by STC but with the American Type designation.
The heater will take a defined time to heat the bi-metallic strip to the point that the contacts will make. Such switches were used to delay the HT voltages until the heaters of the valves were at the correct temperatures. Mercury vapour rectifiers and travelling wave tubes are examples of devices that are protected by thermal delay switches.
The ITT branding and Made in England.
The heater passes through the ceramic tube and thus it takes a while for the bi-metallic strip to heat and bend.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 45 mm tall.
Reference: Data-sheet. Type SH342 was first introduced in 1946. See also1946 adverts.