British Army Electrical and Mechanical Engineering Regulations for the R.106 Mks 1 and 2 (National HRO).  The original HRO (Senior, M, MX etc) although obsolete in the US by 1939 was refurbished and shipped to the UK in large numbers and used for monitoring and direction finding well into the 1950s. 
Compliments of Ron Brown, GW6WRP.