It's an HP 8754A (4 to 1300Mhz) and luckily old enough to be reasonably
serviceable and repairable!
This is just as well since there is a fault with the rectangular trace,
which just appears to 'flatline' with nothing visible other than the
sweep + markers. So no nice bod plots of filters, I'm afraid. However,
the polar and phase displays seem absolutely fine. If I can just get
that one fault fixed, it'll be a great piece of test gear to have
around. I've never worked on anything this complicated before, though,
so it'll be something of a challenge!