EOD - you got the wrong information. This SEL 91 headstamp was the first of the miracle story of CCI’s production of the 9 x 18 mm Makarov, done in a very short time from the request for production until the finished product was coming off the line. The “checked” makes sense, as it was a mark used by the engineer in charge of this production to indicate he had checked the cartridge for defects and compatibility for use in Makarov pistols just coming on the market at the time from the DDR. He would check entire boxes of the prototype rounds and the first production samples. These are not proof rounds. There was no need for proof rounds from an American company for this caliber, as no pistol of this caliber, to my knowledge has been made in the USA. They did make Ballistic Standard cartridges but they have the appearance of the normal CCI aluminum-case ball cartridges - no special markings - and once out of the labeled box, cannot be identified.
It is absolutely a legitimate variation, and a scarce one.
I am not getting into this deeply, as there was another thread on it where, as I recall, I explained the whole thing. I think that is what Hooke was referring too when he asked “again?”
I will let someone who is better at finding old threads reference it here, if someone wishes to do so. I don’t ever seem to have any luck with the search-engine on this forum.