[Jon Skagfeld]

Originally Posted by Chris Suslowicz

That doesn't muddy the water at all: it's the "New Range" of radio sets introduced after WW2 and the letter denotes the power supply/consumption, with the next digit specifying the frequency range and the final digit the set design/model type:

A - low power portable, man portable sets usually primary battery supply.
B - low power vehicular - secondary battery supply
C - medium power vehicular - secondary battery or AC mains supply
D - high power vehicular or static - AC mains or generator supply
E - very high power static installations (usually) may be transportable in sections.

First digit

1-3 HF (2 - 30 MHz)
4-6 VHF (30 - 400 MHz)
7-9 UHF (over 400 MHz)

Second digit is the "design sequence number" of the equipment, starting at zero (low numbers are unlikely to get past trials unless they were re-designated existing kit, such as the E10 (originally Wireless Set No.15, a long range (2kW low power, up to 30kW on HP)).

So you get a slightly more logical arrangement of:

SR A13 (Station Radio A13) - low power HF for where VHF is unsuitable.
SR A40 - low power infantry VHF (also A41, A42 and A43, all different).
SR B47 - low power vehicular VHF set
SR B70 - UHF (really SHF) radio relay
SR C13 - Vehicular HF
SR C42 - Vehicular VHF
SR C50 - Transportable VHF radio relay
SR C70 - Transportable UHF radio relay
SR D70 - a failed attempt as long range SHF radio relay - they forgot the Fresnel clearance.
T D11 - (Transmitter only) Vehicle/static medium power (350W on CW, 100W DSB)
R 230 - Receiver (these were numbered consecutively in order of development)
T E10 - HF Transmitter (58 - 78 kVA input for 2 -> 30 kW output, mind your fingers!)

Chris

Chris: Thanks for that explanation and clarification.