Donald, HP changed from 12.631771.6 Mhz to 12.631772.5 Mhz in the later days. The Zeeman frequency changed to correspond from 42.82Khz to 53.53Khz. The 5071A uses a Zeeman frequency of 39.949Khz. I believe they liked the stability a bit more at the different C-field current. Either way will put you correctly on frequency at the output (5 or 10Mhz) See: https://www.febo.com/pipermail/time-nuts/2005-April/018171.html Read Toms reply and my post below his. The 5071A synthesizer is always jumping around to control the C-field. It's basic frequency is 131.8Khz Basic info here. http://leapsecond.com/corby/5071comb.pdf Cheers, Corby