What Would You Do? Slides Dated in Wrong Order

OK, so this question is similar to the previous WWYD, but involving two pictures, each with a plausible date when viewed separately.

The first has source information written on it dating it August 20, 1973. It is frame 1 on a roll with the description "SEP 73 P 4".

The second has source information written on it dating it June 22, 1973. It is frame 21 on a roll with the description "SEP 73 P 4".

The problem? If both are from the same roll "SEP 73 P 4" then how did a pictured dated August 20, 1973, end up on the frame 1 but a picture dated June 22, 1973, end up on frame 21?

There are several possible explanations that do not involve a time machine. First, the source description (printed on the slide frame by the developer) could be wrong, such as if the slides were put in the frames in reverse order. Second, the source information (written on the slide) could be wrong, such as if the dates were wrong. Third, the roll of film could have been shot backwards, with the the numbering running from largest numbers to smallest; I have seen this happen before on a roll from my wedding (I was very aware of which events happened first, despite the numbering on the roll), and it may be fairly common but unnoticed on old rolls. Fourth, there could be two rolls of film "SEP 73 P 4"; I have seen this before (I will do another WWYD on it) where the multiple rolls are developed in one month (e.g. "SEP 73"), although I honestly do not know what the description "P 4" means.

What would you do?

I did the same thing I did with the mismatched dates on a single slide. I recorded all information from the slides into the file names and gave Grandpa the benefit of the doubt about which dates to apply to the images.

It may be possible in the future to solve this mystery if I get other "SEP 73 P 4" slides. These might show that there are multiple rolls or that the written information was incorrect. Unfortunately, I have only four slides from "SEP 73 P 4", and they do not solve the puzzle.

One thing I will not do -- although it might explain if there are two rolls involved or if there is one roll shot backward -- is dismantle the frame (mount?) and look directly at the film for information. It would be too destructive of the old, paper frame (and, possibly, the film).