I'm going to follow Michael's advice and not worry too much about it. Maybe not the best choice of words, but by 'vertical play' I was trying to describe how the rear standard can move (pivot) very slightly (I'm talking probably less than a mm but still enough to be noticeable) even after focus lock because the L-bracket attached to the rear standard does not fit snugly into the groove along the outer edge of the focusing track. Perhaps it's wear-and-tear or it was never a snug fit to begin with (assuming there's no other adjustment that would lock it down). However, in practice it's not a problem because the focus is set when the back is 'at rest', which it always returns to, despite the aforementioned play, when the film holder is inserted.
Again, it hasn't had any effect in the field just seemed a little odd as all other adjustments lock down tight.
I do appreciate all your help.