There were at least two very different versions of those. One was pretty much a miniature Mauser and one was more like a .22 with rear locking lugs. Back in the 70's I had a friend who got hold of one of the better ones. He was attending a gunsmith school at the time and I encouraged him to make a nice small game rifle of it. Instead, he built up a .308x1.5" on it! What an idiot! He had a very high pressure wildcat round nearly as large as the rifle bolt! And the idiot instructors of the school let him do it! The same idiot instructors who would NOT allow him to rebarrel a '99 savage .303 to .219 Zipper because it might be over pressure for the action, let him build an absolute death trap! I told him "don't even think of firing that thing, you've already destroyed the action, now just throw it away".
To build one, I'd suggest you find the magazine first, 7.62x25 would be the easiest in that regard.