To get an overview: it seems it's necessary to get some software running and hardware wise we probably only need a serial converter.
https://github.com/golanlevin/DrawingWithMachines/blob/main/machines/hp7475a/README.md
Unless they're HP-IB instead of serial. I remember lohxor saying something about the conector.
https://hackaday.com/2022/01/31/save-money-and-have-fun-using-ieee-488/#more-517200
https://www.hackster.io/lightside-instruments/the-gpib4pi-gpib-for-raspberry-pi-hat-4b3e9a
https://alex.nisnevich.com/blog/2018/09/15/plotting_land_use_maps.html
scratch that actually, the schematics for the 7470 differ so much to each other that it's reasonable to believe that there's either not even the same chip in their, or it's programmed with different software completely.
https://www.instructables.com/Modifying-an-Old-HP7475a-Plotter-to-Work-Over-USB/