Perhaps do a little more background checking - for example is the overshoot the same no matter the base frequency of the squarewave
Yes the overshoot is the same at various base frequencies, from 100 Hz to 10 MHz.
Do the other waveforms show any abnormal character?
No every other waveform is essentially perfect from super low frequencies up to their upper limit, that is the sine, triangle, and ramp waves are well within spec.
Do you have another generator that can confirm your 50 ohm load and scope probe etc are working as expected?
No, I do not, couldn't image that it would be that since every other waveform is essentially perfect, even the amplitude and frequency are perfect on every waveform, minus the overshoot on the square wave.
Have you identified the main output amp in the service manual? I'm not sure if the HV output option gets its signal directly from the normal output, or prior.
No, not yet, but that is probably my next step. The HV output takes the input from the main amp and just boosts it, at least I'm pretty sure but I could be wrong, when looking at the RCA connectors on the inside.
I'm almost thinking is may just be a harmonic, because at 10 MHz it is very rounded to which I'm almost certain is caused by harmonics. Still doesn't make sense to me as to why it's such an overshoot no matter the amplitude or frequency. Should I look at the synthesizer circuits since every waveform is synthesized from a sine wave? I mean every other wave except the square wave is fine.
Thanks!