I was lucky enough to find an HP 5061B cesium standard with what I think is a virtually new high performance (option 004) tube. These are rare because the high performance tubes burn up their cesium supply much more quickly than the normal ones. But you get much improved stability while the cesium lasts. I only power this unit up when I need it for a measurement campaign.
When I started this measurement run, the C Field was set to 5.20, which appears to be the original factory setting. When compared to the 5071A, that was a bit high. Some experimentation ended up with a setting of 4.90, which is what these plots show.