I have just checked on my 7904 with a 7B92A timebase. When the 7B92A is displaying A sweep only the readout is in the lower position, but when displaying A and B sweeps together the A speed is in the upper position and B speed in the lower position.
Your 7904 looks very clean but if it has been idle for some time the vertical and horizontal mode switches (A/left, alt, chop, B/right) can get to be intermittent and improve greatly with a large number (ie multiple 10s) of cycles of the buttons. Certainly do this before randomly spraying switch cleaner.
Regards,
Roger