I've been doing gunsmithing on "exact replacement" parts so they can fit exactly enough to replace the originals. I didn't want to spend $250 for the H&K walnut target grip for the PSG1, so I bought the $50 American plastic knockoff. It only took an hour to clean up the mold lines, and grind the relief in the back of the grip so it fits like it should.
It was actually quite calming, except for muttering about quality control. When done, I had a 17+ pound example of late-40's design made in the late 70's, with late-90's plastics.
The folding stock comes in handy in a couple of different ways. First, since the Smith Industries vortex flash hider is 3 inches longer than stock, the rifle would not fit in its case any more with the fixed stock. Also, the shape of the stock lets me use a "cheek weld" on the iron sights, and a "chin weld" to see through the scope.
A definite "machine age" weapon, as opposed to the "information age" PS90. But the H&K-91 has a purpose to serve. Carrying it several miles is not one of them, in my book.