I thought of both of those possibilities, but I don't think they can explain these images.
1. For the dark colored positives I don't see any evidence of the dark color bleeding through the light halo. Also, some of the positives have fingers that are faded, and some are overlaid by negatives. A light halo overlaying a dark base layer can't account for those.
2. The positives are too solid to simply be handprints. If you just make a handprint, it will have gaps in the center of the palm and between the palm and fingers (among other places). But all the images are solid silhouettes.
1. For the dark colored positives I don't see any evidence of the dark color bleeding through the light halo. Also, some of the positives have fingers that are faded, and some are overlaid by negatives. A light halo overlaying a dark base layer can't account for those.
2. The positives are too solid to simply be handprints. If you just make a handprint, it will have gaps in the center of the palm and between the palm and fingers (among other places). But all the images are solid silhouettes.