The vignetting or whatnot on photo #17 is defiently odd. ...
That's not vignetting, and I think that's a clue to what these photos really represent ...
Vignetting refers to a gradation of brightness or color saturation applied to direct a viewer's focus to the center of a picture.
https://en.wikipedia.org/wiki/Vignetting
There's a distinct circular border around these photos rather than a graded fading. I think these represent circular matting or cropping on a still picture.
Possibility #1: These last 3 1969 photos were taken of 3 hardcopy (perhaps framed) photos that were matted or cropped within a circular border.
However ... I don't particular like this option #1, because it would be odd to have 3 photos of what appears to be a single gathering or ceremony, at least two of which seem to portray a sequence of events.
Which leads me to Possibility #2 ...
First, a brief side note ... I don't think the figures in the photos are Native Americans. Look at the hair and attire. The hair makes me think African or Polynesian. The attire tilts me toward Polynesian. owing to the frilled trim of the 'vest' (vestment?) and collar visible in photo #17.
Now ... What would be encountered as a set of photos illustrating a sequence of events involving some exotic indigenous culture from another continent?
Possibility #2: These 3 photos represent halves of 3 vintage stereoscopic slides or photos. Such circular cropping was not unusual for stereoscopic slides or viewers. I think the circular cropping resulted from using the family camera to snap an image through one side (i.e., through one lens or view-hole) of a stereoscopic viewer.