I finished refurbishing a 1949 Jackson RF Signal Generator and attached a Chinese frequency counter to it. For such a vintage instrument this Jackson is pretty darn good. it settle down real fast and I am looking at the digital counter at 455 kHz only the "dot" XX numbers are gyrating, some what more gyrations in the mHz ranges but over all I think the Jackson is close enough.

A little tip: Most bench signal generators that had an AM audio tone modulator were not designed to produce 'clean' AM modulation. Most produced AM modulation with some FM modulation content as well (which is handy if you need to use one to service an FM radio), thus if it allows you to turn off the modulation you should see the frequency reading further stabilize.

It has two switch positions on the left side, Modulation, and CW. If I switch it to Mod' the frequency display makes no sense at all.