Seeing as there is a 74 series high speed logic IC there, that more than likely is doing a function of high speed output buffer, and driving of the output coax. The IC being there with 2 capacitors suggests it is being used as 2 stages of buffer for the oscillator. Thus the IC will be connected to the oscillator, not the output, and it is giving a 10MHz square wave output, instead of the low drive ability of the oscillator one.
I would lean to the other rail being a 10V one, as this will at least be high enough to power the heater fine, and the oscillator having 4 pins puts one being ground, one 5V supply for the oscillator in the can, one pin probably is connected to a resistor divider providing some trim for the oscillator, provided by the precision resistors on the board, and the last one is a heater supply.