Overview of fence types

[L4R3L2]

The C87 is one friction fence lock that benefits from a left hand or vertical up orientation. If (or when) the clutch wears down, those are the only positions where gravity can assist to open.

[MartinHewitt]

The patent of the movable fly! https://patents.google.com/patent/US821548 So now we know it's from 1904/6.

Just found the patent https://patents.google.com/patent/US370568A from 1887.

and it is preferable to introduce between the permutation-wheels the flies 9, Figs. 3, 5, and 6, that are formed with eyes passing around the tubular stud or around the hubs upon the tumbler-bodies, and have a limited movement, so that the studs 10 upon the tumblers may stop in line with each other when setting the tumblers. These flies are well known in this character of lock, and may or may not be used with my lock.

So it looks like flies were old tech already 20 years earlier. Maybe they were not used due to patent issues?