I just purchased 3 A2 apertures to replace a couple of A1s along with the horizon line aperture on an ARMS #40. They are marked "1S", ^3 (arrow pointing up and 3), and "H1'. The 0-2 markings are different fonts on each. One, the "S1" example, is noticeably higher than the other 2. Are these USGI apertures? Are they cast or otherwise not authentic? The company I bought them from claims "all US made parts."

The S1 sounds like an A1 aperature, the others are probably A2.

They are all A2 apertures, but the fact that they are not exactly the same in dimension and the fonts for the numbering makes me curious???

From the Maryland AR15 site:


Q: What is the change of impact between the two apertures in the A2 sight?

A: Courtesy of XS Sights from their website (http://www.aosights.com/news.html FAQ #10)

...Comparing some of the rear sights available today there appears to be 3 variations available. The 1st style (and oldest) has a .014 offset. This gives a 2.52 inch shift in point of impact at 100 yards between the two apertures. The 2nd style has a .017 offset, which shifts point of impact 3.06 inches at 100 yards. The 3rd style has a .024 offset giving a 4.36 inch shift in P.O.I.

All of the points of impact shifts shown above are for the standard barrel AR-15/M-16 that has a sight radius of 20 inches. The shorter sight radius (14.5 inches) of the M4 carbine and weapons of similar configuration increase the amount of point of impact shift. The shifts for these shorter sight radius weapons are 3.48, 4.25, 5.96 inches respectively....



Maybe that will help.