I feel almost cheated. The only things I needed to do to get plotter working was plug the power cable and turn it on. That's a rather rare case. But there are still challenges to overcome.
First of them is communication with PC. Plotter has two connectors: one for serial (25 pin) and one for Centronics. I didn't had any of them at hand, so I needed to look for something in a pile of junk which I luckily have available at work. I wasn't looking for ready cables, expecting that internal connections probably will not be as needed, so I ended up with 25 pin cable with DP25 male connector and a DP9 female connector kit. Perfect!
Now I just needed to solve puzzle of what cable connect where. I found connection diagram for HP 7475A plotter, which as I believe was an "inspiration" for my device, and compared it with female DB25 connector diagram in manual. It was a match. So I used a multimeter to find out which cables are connected to the communication pins in DB25 female connector on cable. This was a tiny bit tricky, because one need to remember that the pinout on cable connector is mirrored versus to this drawn in manual. I made myself small tags with numbers to remember about that.
I needed to connect only 9 signal cables (plus shield), so the remaining 16 cables were cut short. Soldering was easier than I was expecting thanks to the third hand and tweezers. The only tricky parts was when one cable needed to be connected to two pins on DB9, but I made a small jumper and it worked. The cable as ready.
But how to check if it works? After some research I found that this plotter uses HPGL for communication, and Inkscape cane export such format and even drive serial plotters. Success! It worked from the first time. The only bad thing of this situation is that I was not prepared for that and I don't have any pens available. But this part of the project is done.