Since the date codes on most of the ICs indicate 1989 or 1990, I searched the HP catalogs around those years on the
HP archive, but could not find any single reference to the HP boards.
I was able to find only a reference for the 37288A on a
Keysight page that says the product is obsolete and there are no documents about it available.
140Mb/s is the fourth level of the E-carrier system in a PDH (Plesiochronous Digital Hierarchy) network. It was used primarily in Europe and some other parts of the world. Perhaps those boards belong to a specific catalog for those markets. It is a multiplexing system for digital telephone lines, replaced by SDH (Synchronous Digital Hierarchy) and Ethernet with IP switching.
The use of 75-ohm cable is because it offers less attenuation. A 50-ohm cable is more suited for high power transmission.
The signal coming into the 37288A, probably coded in CMI or HDB3, must be treated by an amplifier with AGC, then handled over to clock detection and pulse regeneration. This may explain the presence of that hybrid IC (perhaps a PLL) and the presence of the delay lines to adjust the exact timing when 0s and 1s should be decided.
These networks had strict timing and level requirements, didn't allow retransmission of a wrong sequence, and demanded a very low bit error rate (BER) in the range of 1 error per 10⁹ bits transmitted max.
Those were the days.