When and where this type of fly was developed, I'm not sure. I'd guess the fly in general was a Germanic development given their use of rifle and set triggers. The Germanic lock on RCA #42 (1760's likely) has a fly that wraps around the small tumbler shaft and when I built a copy of this lock, I included this feature. It's a set-up that works quite well. Seems to be a little more substantial and stable than the typical little flies used.