You can find the manuals online at w140.com/tekwiki or Google for 'BAMA manuals'. Artek Manuals have very high quality scans at modest prices. The 7904 and 7904A are very different.
There are separate adjustments for centering the character readout and the vertical signal. If you want to check the centering of the mainframe vertical amplifier without any special equipment just remove the vertical plugins and turn the scope on with the timebase set to auto trigger.
If you don't have plugin extenders you can get some access to the left side of the vertical plugins and the right side of the horizontal plugins by removing the main side panels, but you need to take extra care to avoid shorts to the metalwork.