Presentation Copies

On several of the auction sites, I've noticed 19th classic US items described as "presentation copies." These invariably have two colored lines one vertical and one horizontal forming a cross on the stamp. Initially I thought perhaps they were a kind of proof, but all seem to be perforated and look no different from the issued stamp except for the cross. They seem to serve some specific purpose, but I'm at a loss as to what it could be. No doubt someone on SCF has the answer.

Could be copies, fakes or forgeries that have been marked and given to interested persons for research and reference?

They're legitimate and I have seen them before. They was overprinted with the red lines so they could curtail their use as postage. They usually was given out as gifts, mainly post offices upon request.

edit:
I'm sure that, like anything, there are fakes made of these. I would really be cautious buying one without knowing for sure what to look for.

ILS is right on the money with his caution, and with his comment about the legitimacy of bona fide presentation copies. In their time they served roughly the same purpose as Specimen overprinted stamps, but in most cases a presentation cancel is in pen. They are occasionally expertized, but more often it is a matter of knowing the provenance or the parallels to an item of known provenance.