Good idea. I would have suggested it but I didn't know you hand another
timebase. If you switch the CH2 Polarity to Invert it should switch the
angle of the pattern 90 degrees. Alternatively you can stick a few
microfarad capacitor in one of the two signals going to the vertical plugin
to generate a phase delay. It will lower the amplitude and you will probably
have to increase the frequency a few orders of magnitude but you will
eventually get an oval at some combination of frequency and capacitance
since the capacitance is in series with the 1Mohm of the vertical amplifier.

I don't know what relays Tek used in the 7904A but often they are in
sockets. If they are in sockets and they are the same try swapping them. If
they are in sockets and they are not the same try removing them and applying
a voltage to each coil and listen for the click. The coil voltage and coil
contacts should be marked on the relay. If they are reed relays try holding
a magnet next to each one and you will hear a very, very faint click as the
contacts close or open.

If the relay's only have a Tek part number on them someone on the user group
will be able to tell you what the pin outs and coil voltages are.

Dennis