I will probably dig out a few traceable passives and use them to do a check that the thing is reasonably close, if it is, then fine, if not well I have a service manual, and old HP is generally really serviceable, failing everything else it goes back on EBay as a 'parts only' job where I got it from.
This is not intended for metrology, but as a working tool for winding RF inductors and such (And maybe doing G-B plots of sonar transducers), so if it is within a few percent, that is good enough.
Short, open and measure something known in the range that I am interested in, not calibration, but reasonable verification, far more important most of the time.
First job, as always, is dig any RIFA class X or Y caps out of there.....