Kraig, that sentence in the CMP rules "A corrective lens or multi-focal lens system may be inserted in the rear sight hood." was a result of an incident in the President's Hundred match some time back in the 1980's at Camp Perry.
A good friend of mine was shooting his M1A with a -.375 diopter lens super-glued inside the NM rear sight's hood. He was talking about it with his scorekeeper and the Referee just happened to be walking by at the time overhearing him. The Ref immediately declared the rifle illegal for DCM/CMP matches and that old retired Navy MCPO using it quickly challenged it. His reasoning was, in order to allow older folks with less than perfect vision compete equally with the younger, sharp-eyed folks, they should be allowed to use it. Filling the aperture with plastic steel then drilling it out to .040" to sharpen vision was legal but it darkened the target image too much. Besides, nothing in the rules specifically mentioned it not being allowed anyway. He kept on shooting the match under protest or something like that. So the Match Rules Committee met and talked about it. They all said the rear sight looked like a standard NM rear sight and older folks should be able to see their sights clear and sharp just like the younger ones. The next year, that rule sentence (above) was inserted. And the guy using the corrective lens placed in the top ten of the match.
US Navy Distinguished Marksman Badge 153
Former US Navy & Palma Rifle Team Member
NRA High Power Master & Long Range High Master
NRA Smallbore Prone Master