.

I was not so much addressing the geometry as the visible construction.

It appeared to me that there were a great many small dark imperfections at the root of the square thread, almost like chatter marks or casting voids, and it was those small points that I was referring to as potential stress raisers.

I have not calculated the loss of engaging-face area between the square contact faces of plug and bushing that would result from 'rolling' the root of the thread in the plug to a sensible radius (and relieving the nose of the bushing threads to clear). But I can't imagine it would be more than a few percent, and of course at the distal edge of where steam under pressure comes in contact with the sealed plug assembly. Surely this is a reasonable design change to consider?

I agree with all the other points made.