STV was the trademark of the Stabilovolt company of Berlin. They are mainly known for their range of Stabilovolt voltage and current stabilisers.
Stabilovolt became a division of the Telefunken company. The date is not known but during WWII their products carried the Telefunken brand. See 251 and H85255/80.
Stabilovolt was a name seen on Marconi adverts and it is probable that Marconi had the marketing rights in the UK.
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