Sorry about the confusion of the test setup. The 7704 has 2 7A26's installed in the verticals.
I understood what you had. Once calibrated, you should be able to exchange the 7A26s and only have to adjust their front panel gain calibration.
If we think I can call the PG506 good I can start tuning these as if I can trust the PG506 these need a bit of a cal. Pictures follow.
I think you can trust the PG506 but the only way to know is to check it against another known good oscilloscope or use one of Leo Bodnar's pulsers which I linked.
Below are a couple of photographs from my 500MHz 7904 which was a design contemporary to the 7704A and released at the same time.
The first two photographs are a random 7A26 plug-in with my PG506 driving both traces. The third slightly smaller trace on the right side is my 200MHz 7000 mainframe standardizer which gives some idea of the transient response of the mainframe by itself.
The third photograph is with a dual trace 400MHz 7A24 (300MHz in the 7904) vertical amplifier which has 50 ohm inputs at two different sweep speeds. The PG506 is not really fast enough for this configuration.