Here's the plugin module PCB layout, gerbers and rear panel design files.
I pulled the module from my instrument, desoldered the parts, scanned the PCB (then reassembled and reinstalled) and drew up the PCB design.
I still have to back-annotate to a schematic, but I'll do that later.
This is a 100% functional copy, and almost 100% cosmetic copy too. I just used standard footprints in Diptrace for the passives, so their spacing is inconsequentially slightly different to the HP design, but it makes no real difference.
The brackets for attaching the PCB to the rear panel will need to be fabricated. I haven't altered the design there at all, so it currently matches the HP design exactly.
If an easily available part is decided on, I can update the files to suit.
All the IC's are 74LS series, but they should be able to be replaced with 74HCT without any trouble.
Also, the Centronics connector could probably be replaced with a PCB mount type to ease assembly by eliminating the need for ribbon cable jumpers if we wanted. Something like NorComp part 112-036-213R001 would do it.
A minor change to the PCB and rear panel would be needed but nothing too difficult.
The BOM is as follows:
IC1 - 74LS245N
IC2 - 74LS244N
IC3 - 74LS164N
IC4 - 74LS00N
IC5 - 74LS244N
Resistor Arrays (x2) - 4.7K 8-way ganged - Bourns 4609X-101-472LF
Resistor - 10K 1% 1/4W
Ceramic capacitor - 0.1uF
Electrolytic capacitor - 220uF 25V
Diodes (x2) - 1N4148
Centronics Connector - NorComp 111-036-203L001
As for the control box, I don't have one to copy, and I don't have a copy of the user manual for this option either. I posted a request for a manual on the HP/Agilent/Keysight Groups.io page but no response yet.
I see manuals on eBay, but the sellers are asking like $50 or more which is a silly price....