speculated it was for covert ops that required no evidence at the scene
That's just silly.
ANY spent shell casing at a crime scene is excellent evidence. The boltface and extractor always leave unique marks which are far better than rifling-marks for the purpose of tying the casing to a particular gun.
A UNIQUE headstamp (i.e. absence of markings) is every bit as much an identifying mark as if it said CIA or KGB on the case.
The spent case would contain gunpowder residue. Almost all Gunpowders all have taggants added to the powder, so it can be identified which manufacturer produced the powder. Even ball-point pens use taggants. Absence of a taggant would dramatically narrow the list of suspects.
Even in the absence of taggants, a gas chromatograph would provide a chemical "picture" of the gunpowder, such that if a second casing, lacking a heastamp, and with the same breechface marks, and containing the same very unique chemical picture, and absence of taggant, would ring the bell as to identifying that the second casing came from the same gun as the first.
All this James Bond stuff is very real, and in play today. Well, unless you are dealing with insurgents, who will probably identify the shooter by his feces.