Very nice tear down and functionality check.
The time base selector has a 10 nsec setting, therefore I assume the internal time-base PCB to deliver 100MHz, not only 10MHz.
The circuit contains a rather simple 10MHz oscillator on the right, but there's that MC3060, which might be an ECL dual FF, similar to 74H74, so inside that metal can part might be an ECL PLL circuit (int and ext) to 100MHz.
There's also that -5.2V supply, which indicates more fast ECL ICs inside the counter.
So maybe the inputs work up to 100MHz also.
Obviously they use two distinct mains transformers, one permanently operating for the OCXO, and the bigger one, switched, for the counter itself.
I also ponder about these both osc. inputs on the rear, ext./int Time Base and STD Freq Input, where to feed the OCXO 10MHz in. I did not catch your description in the video correctly..
Anyhow, that TTL grave reminds me of my own counter build in the 70 ties, but already with the higher integrated Mostek MK50398 counter / latch / display driver and 7 segment LED.
It had a 7th digit, for > 10MHz counting, and a period mode also, to get higher resolution for low frequencies, by dividing either input frequency, or time base / gate time.
That's obviously also provided on the Systron Donner counter, when you use B-input and period mode.
Frank
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