You know most frequency counters have a 10.000 MHz outlet on the back of the cabinet. I have a cheap frequency counter that has the 10.000 MHz outlet on the back. I opened it up just to inspect it since it was a eBay purchase. The 10 MHz is a top quality reference oscillator in a temperature controlled oven. I took it to a local calibration lab and had it all checked out. No adjustment was required since it met the manufacturer specifications.
I asked if there was any error in the read out and they said it was less than .5 Hz difference. (More warm up time?)
Close enough for any standard in my book.
If you use any item for a transfer standard you have to have calibrated twice as often as the items you calibrate. I guess that's a mil-spec thing.
I did a lot of calibration work in the military and it seemed like every eight weeks we took my station apart and down to the base calibration shop.
I have the B&K precision 1040 and the 2040 units(Calibrated) and like that setup. I'm still making cables to hook it all together.