It has a couple of additional features. In the top of the mandrel are 3 holes. The two outside ones are threaded, the third runs through the hole that can be seen at the top.
It suggest that there was a top piece which could be screwed onto the mandrel after it was inserted in the frame, probably to lock it in place.
I made a little wooden reproduction piece using 2 screws with identical threading, just to see how it would work. Even with this small wooden retainer, the frame and mandrel are tightly locked together.
The visible hole at the top side lines up with the magazine button, I think it can be used, with another additional part, to simulate the activation of the holdopen as well. I tested it with a magazine button sized screw head stuck in the side, and indeed, it activated the holdopen.
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