What Does this NYA measuring tool measure?

The basic tool. It is obviously an outside caliper. 16cm long. I have no idea why the jaws are triangular and it must be noted that the inside surfaces are square and parallel but the outside edges are not. There is a measurable taper getting smaller toward the tips.

Here it is again. This side is marked 'METER' and measures centimeters. So far, so good . . .

This is where the fun begins. What the fsck is an NYA and what would one measure with them? The next two pics are crops of the one above.

.720"

.739"

.750"