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It is important to realise that there were three quite separate 'VT' series.
A British Post Office series (VT = Valve, Telephone); sometimes referred to as the POVT series.
The RAF series (VT = Valve, Transmitting); also VR, VU, VS, VI, etc. series.
The WWII US Government series (VT = Vacuum Tube); roughly comparable with our CV series.
Each of the three series started at VT1 and ran in parallel but with entirely different types of valve! There are thus three completely different types of VT25, for example.
Data on the (PO)VT series is particularly hard to obtain; it was the oldest of the three and it is suspected that additions to the series from the later 1930s onwards were 'harmonised' with the RAF VT series. See VTno.25
The RAF series is well known in the UK but data on types without civilian equivalents can be hard to find. This series ran up to around VT130 and then re-started at VT500.
The US series ran to around VT300. Most of these are equivalents of standard civilian types but made to US Government specifications.
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