The letter being revered is a way of indicating if the beam went from the back to front (posterior to anterior) PA or front to back, AP.

X-rays are effectively shadows and things further from the X-ray plate get more magnified. Large structures like the chest benefit from being done a certain way, like having the heart near the plate. That way it’s less magnified.

PA or AP is often a thing you see on the corner of films, indicating the technique.

The film also used to be processed in a film processor, and it was very easy to accidentally flip. This is another reason we have conventions.

The surgeon has no issue with it because it is essentially universal policy in the US (don't know about other countries but can't imagine they don't have similar) that the surgeon must, prior to sedation, mark the surgical site themselves and put their initials on it. Exceptions are made for cases where there isn't a "side" (you only have one gall bladder), or where the wound is "obvious", or for exploratory surgeries where the actual site of all injuries is not yet known.

I can add to these stories because one time I needed to overrule my doctors opinion and use a digital interface to point where the problem is. The peoblem involved X-ray.