This kind of test gear was an absolute must for companies producing transmission telecom equipment back in the day.
Not only HP made them, but also Rohde&Schwartz and Anritsu.
Because in the Plesiochronous Digital Hierarchy (PDH), the tributaries of a multiplexer are almost (Plesio-) but not synchronized, stuffing bits have to be added to the fastest tributaries so that their bit rates are lowered to accommodate the slower tributaries.
Since this operation is statistical in nature, it adds a "random" jitter to the pulse train, which translates in errors and, ultimately, in noise at the reception.
These companies had to comply by law to the CCITT standards, otherwise they could not sell their stuff. So they needed a reliable jitter analyzer to guarantee an absolute compliance before certification.
This construction pattern of a motherboard and several plug-in daughter boards was the standard of the era not only for the test equipment, but also for the very telecom equipment for which they were designed.