It’s definitely a reel change. I’m not sure why they sometimes appear to be handmade scribbles, such as that, rather than a clean oval.
As a projectionist, if you end up having to splice the film near the reel end and the result means the frame with the cue-dot on it is now missing, you have to put ‘something’ there as a marker for the next projection of the film so that you know when to switch projectors to the next reel.
Some guys used a hole punch, some would do a scribble or some kind of mark to replace the missing one.
I haven’t seen the scribble mentioned here specifically, but it is possibly the reason it is there.