I noticed that on a MaTech the concave (dished) side faces the rear, on an A2 the small apperture dished side is to rear, but on the larger apperture the dished side faces forward. I have an SP1 and the front apperture has the dished side forward. Rear apperture is drilled out, no "L" mark. Is it installed backwards?
I'm assembling some uppers soon and the DPMS A1 sights don't have L marks, wich way should the dished side face?

I could never find an answer one way or another.

The concave side of the small aperture on the rear sight leaf of Colt (mil-spec) A2 rear sight assemblies was designed to face rearward (towards the shooters eye.)  The concave surface of the small aperture was designed this way to help prevent the finish from wearing off and thus leaving bare metal to cause glare to the shooters eye.