I'm not (yet) convinced this is a set of actually affiliated images, much less that they represent one or more incidents observed from the
Trepang in 1971. Here are some reasons why ...
The stadia lines (crosshairs, tick marks) visible in these purportedly through-the-periscope photos are notably crude, uneven, and in at least one case doubled. I dare anyone to rummage through the relatively plentiful inventory of WWII USN periscope view photos from 3 decades earlier and find any in which the stadia lines look as fuzzy and crude as in these photos. In some of the photos the lines look as if they were drawn over the image using a pencil or pen.
The second photo shown above has to be a double exposure. The vertical crosshair seems to split (as if overlain with a second impression rotated slightly clockwise). This cannot be an artifact of the periscope's rangefinder (which could split an image into two parts displayed one over the other, but not add in any such rotation).
The
Trepang, as a
Sturgeon-class attack submarine, was equipped with quite sophisticated optics (a Kollmorgen 15D unit, to be exact). The Kollmorgen 15D had built-in mounts for a camera, and use of this integrated camera wouldn't have resulted in the sort of fuzzy shots presented here.
There's no explanation as to where the 2 color photos (scanned off a magazine page, as evidenced by the staple holes) came from. I would also note these are the only photos in the set which show no signs of stadia lines (and presumably were photographed 'top side').
Not all the 'cigar object' photos show an object of the same shape. In the clearest photo it appears to be hemispherically convex on one end and flat on the other. The color photos of an elongated object indicate both ends are bullet-shaped.
The ambient weather, sea, and lighting conditions don't correlate across all the photos.
The triangular object(s) resemble similarly triangular spots or smudges visible in other photos.
The 'big splash' photo:
... seems to indicate (via the splash pattern) the object is erupting upward rather than crashing downward.
Trepang didn't have a deck gun, nor was it equipped with anti-aircraft weaponry. If the mystery object was indeed shot out of the sky, it feel victim to either (a) small arms fire from someone atop the surfaced sub or (b) some other party's fire.
The 'big flat, vaguely triangular object on the horizon' photo:
... Looks to me like a (unrelated?) fata morgana / mirage.
Finally, some of these images appear to have been viewed while the sub was surfaced, while others could well have been viewed from periscope depth (submerged).