I know you are looking at the picture and thinking, that is not so rare it is a HK 91. Well actually what you are looking at is a HK 41. Brought into the US in the 60s, this is one of those rifles that once it got here the BATF decided was way too easy to convert to select fire. This one is dated 1966. Note the pin at the point the trigger guard meets the mag well. That is what bothered ATF. A G3 lower and bolt would go right on this... So HK was told to go back to the drawing board and modify the rifle.
The magazine article is from Shooting Times in 1963.
Had a HK 43 at one time too. I foolishly let that go.