It could possibly be orientation as defined in the xdp. Try messing with the pitch, bank, and heading variables.
Gunsight Controls:
I have no idea about sun screens either, just wanted to throw the idea out there as many sights had them.
Adjusting angle - could this be done by modifying the pitch value set in the xdp by adding, say, 6 degrees to the value? Changing the pitch does change where the aiming point is, as I found when setting up a sight for the upward firing guns of a Bf 110. It was a bit harder to get right, but it works perfectly.
Materials:
I take back my previous comments on the specular strength, I think you've nailed it. Some of the specular maps are adjusted too high, and I need to darken them, but when comparing some period color photos to an aircraft using your shaders and having a properly adjusted specular map, the strength and appearance seemed right on. Another factor in my initial observations may have been the glossiness fresnel effect seeming to increase the specular.
I have yet to try bump mapping, but hope to soon.
It seems the fresnel effect and maybe others are only applied to aircraft exterior models. Is this correct?