I could suggest, maybe just get a new rear sight base, it is an A2 right?

Have You shot it? Other than the rear sight problems, Does it hold decent groups?
How does the barrel and throat look? Is the rifleing in good shape? does the throat look eroded? Do you have access to headspace gauges?

If everything looks good, and its just the rear sight, try a rebuild or replacement.

Chances are, If You can't find any marks on it it's way down on the food chain, and whoever built it may not have a warrenty or, a very limited warrenty on it.

Try this first, Flip up the rear sight, so that the long range is up. You'll see a hole in front of the sight. Take a 1/16" allen wrench, put it in the hole and loosen the screw. LEAVE the wrench in place, rotate the bottom of the sight in the directon of the 3/8 mark, going off mine try about 7 clicks. tighten the the screw, the see if the 3/8 mark is in the right place, it should be bottomed out. if not just do it again.