[Julesomega]

Thanks for that link Richard, and hooray for Mr.Cain! The issue was devoted almost entirely to the TF2000/2100 and the completely unknown "MF" variants which extend the range to 550kHz (they were not in the later catalogues I have). It's hard to imagine what market these 600Ω instruments were intended for, and carrier telephony systems must have been into the MHz region by then