@robertusa Cap is part of the monument, a bit different than an accessory, but doesn't make the monument any more or less valid depending upon the cap's presence or absence.?ÿ The purpose of the cap is to identify the licensee who is responsible for it's placement, and sometimes to hold other markings designed to more particularly identify which corner or reference position it is intended to mark.
I've found old iron pipes of the type described in GLO or BLM notes but whith the cap appearing to have been pried off by some souvenir collecting geocacher.?ÿ I didn't for a fraction of a second consider that the pipe remaining in the ground did not mark the corner or that it was any less valuable as a monument.