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problems (backwards compatible), but the reverse is not always true.

Anyhow, James gave this bit of info:

In reply to your Time-Nuts question.  The smaller PCB was used in early
equipment (such as the HP5328A counter), with the 10544A oscillator.  It has
just the basic elements to support the oscillator; essentially a voltage
regulator and a little filtering here and there.

The larger board was introduced later and included the above, but also extra
stuff to provide:

* Automatic detection of external 10 MHz source
* Buffered 1 MHz and 10 MHz to the external world.
* Support for 10544A and 10811A oscillator (little difference electrically)