Referring to this study, you'll see that the ends of the loop were sometimes milled away in final shaping...
http://forum.lugerforum.com/showthread.php?t=24367
The end of the loop can be seen in some of the photos. It's possible that it was peened and flattened to fix the loop into the frame, and then the milling done to smooth the entire area.
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Igitur si vis pacem, para bellum -
- Therefore if you want peace, prepare for war.