@Sun City,
I pulled out my Davis lock and took the bridle off to show some visibility into the function of the fly.
1. Hammer down. The Fly is free to move in the mortise in the tumbler. The half **** notch is uncovered.
View attachment 99004
2. Half ****. The nose of the sear has pushed the fly out of the way so the nose of the sear can enter the half **** notch.
View attachment 99005
3. Full ****. The sear has passed over the fly, forcing the fly toward the half **** notch, covering the notch and presents the face of the rear bevel of the fly to the nose of the sear.
View attachment 99006
4. Sear is released and has pushed the fly to cover the half **** notch and because of the bevel in the fly, the nose of the sear will ride over the half **** notch as the tumbler is rotated by the mainspring. The sear is held against the tumbler by the sear spring. If the fly was not there then the nose of the sear would enter the half **** notch.
View attachment 99007
You can see the differences in the front and rear bevels on the fly. The rear bevel has to extend slightly below the face of the tumbler. See photo 1.
There's no way to turn the fly over.
In the final picture, I have installed the bridle, the lock is in full **** and the fly has been pushed forward to cover the half **** notch.
View attachment 99008
I hope these pictures clarify what is happening with the fly, sear and tumbler as the hammer is falling.
When the triggers are pulled unset, the nose of the sear is held away from the tumbler by the front trigger lever and can not enter the half **** notch.
Now I am waiting to be chided for leaving all those wood chips and dust all over the lock. They will be cleaned ut when I get to the next stage of the assembly of the rifle.