Brass catcher on the K was strictly a Swedish thing. Never saw one in any US Government, military or Embassy armory I was ever in that had K's. The Green finish was listed as "Tropical" finish. Those lucky enough to have a Wilson Arms (GA) marked green Swedish K with fixed mag well registered with serial number engraved on the stock release lever would have one of the so called "sterile" Swedish K's that saw issuance in SE Asia (not necessarily in Vietnam).
The rarest accessory I ever saw for the K were the sanitized magazines that were reparked by the US for issue to SOG and SF. Seems the K guns bought from Interarms came with a nice assortment of magazines and in addition to the Swedish markings on the back of the mags, they were also marked in english with "Made in Sweden" on the bottom right side of the mags.................before being issued, they all were run through a machine shop and had all the markings milled out, then given a nice park job. I know this since the work was done by American Machine and Foundary (AMF) Firearms/Special Projects Division in Alexandria, VA in the early 1960's.
A very close friend of my family worked there and had an assortment of handguns done by AMF or certain intelligence agencies. Two of the most notable were a real nice Belgian Browning High Power with all the markings neatly removed and given a nice, dark parkerized finish. When compared to a issue BHP, you could see that the slide and frame had been narrowed on a mill/removing the slide markings and serial number. The other was a silenced Luger pistol that looked like a Bull barrel target pistol, but the silencer was built under the tube.
AMF did this kind of special work from 1960 until about 1971. Their factory was leveled several years ago, and all the old hands there have long since passed on. The BHP and Luger were turned in by a fairly ignorant son of the AMF employee, since he believed that his late father was some sort of "Assassin" when he was in reality an engineering genius.
Still, the Swedish K is a great SMG. I think there are some classic pics of the AMF silenced Swedish K on line and in the Archives. The other AMF silenced SMGs and Pistols are less well known.