Another hypothesis: this is a retouching layer that would be combined with the camera negative. It would adjust the tones, remove scratches, maybe add some mood/expression, extra texture, etc.
Just like Photoshop. That way you don't risk damaging the original negative, and you have more options to work the image with aerograph, paintbrushes, etc.
I suspect the reason why you don't have the eyes is that they are on the camera negative, and usually the point where focus is set. So you print the negative first (probably lightly), then you lay on the retouching layer over the image and re-expose. Or maybe you need to do an interpositive first, and then you-recombine, etc.
It could be for darkroom use, or maybe for pre-press work. Retouching could get quite sophisticated in the analog era.
Still, a very cool artefact.