I measured two more of them and got 47.999612MHz and 47.999745MHz so they are quite consistent. One thing you have to watch is that the FM setting on the sig gen needs to be carefully set to keep the HP8405A within the linear region of its phase detector on the selected phase display range, i.e. to keep close to zero. I had to change the FM setting on the sig gen quite a bit up at 48MHz.
One way to check the validity of the test is to turn off the FM modulation and close the shorting bar and adjust the VVM for zero phase at 48MHz. Then release the shorting bar and manually adjust the sig gen in 1Hz steps until zero phase is achieved again on the HP8405A. The frequency should then be the series resonant frequency. Then repeat the shorting bar zero cal at this frequency and double check that the phase is still at zero with or without the shorting bar. This proves the phase is zero for both the crystal and the shorting bar at this frequency and so it must be the series resonant frequency of the crystal. I didn't try using a resistor to emulate the crystal ESR here, I just used the shorting bar. But I got good agreement using the automatic FM/loop method and the clunky manual method. So I think it is all good
