Kenneth scored two HP universal frequency counters and popped them open. One generates the timebase from an uncompensated oscillator in a TO-5 package, while the other uses a large temperature controlled oven oscillator:
here is both of my counters measuring the same 1pps taken from my AdaFruit GPS receiver. The lower one with the oven-compensated oscillator (which is marked as being calibrated in 1999) is spot on, while the top counter using an unprotected crystal oscillator drifts up and down 0.1-0.2ppm, and is almost 5ppm slow…The standard time base is rated for 2.5ppm drift between 0 and 50 degrees Celsius, where the [oven oscillator] time base has the much tighter 0.007ppm. Since these counters display eight (or nine) digits, 2.5ppm is very visible, where 0.007ppm will at worst only effect the least significant digit.