The square things look a lot like brass or
bronze keys for shaft and whatever it is that mounts on the shaft. But being the same length as the
aluminum pieces makes that unlikely.
The circular things with small ears are dogged washers if they're flat, but looking closer makes it look like they are ends to the
aluminum pieces, and the square-holed washers suggest that the 'keys' go in them.
A
single chamfered hole suggests that should be held down with a flat
head screw; why there are two non-chamfered (countersunk) holes is a head-scratcher, though.
Is this the entirety of those sorts of pieces?
Having the manufacturer's name on the box isn't necessarily instructive; only 3 pieces is not very many, and if there is nothing on the boat which looks like it, it could be entirely unrelated to the boat.
Those pieces also look like they have not previously been installed (no wear marks on the holes).
The small wing out the side (110° or so off the larger edge) makes it unlikely that the large edge would be screwed down to a flat surface (large edge would not lay flat).
Good mystery!