Like galamb said, the L&R locks fly can be put into the tumbler two ways.
One will work but if the fly is turned over and installed, it won't.
It's been a while since I checked out which way it the right way but the fly in your picture looks to be right, I think.
I'd suggest assembling the lock.
Then, before putting it into the gun pull the hammer back to full cock.
Hold the hammer to keep it from falling and release the sear.
Allow the hammer to slowly fall, watching the nose of the sear and the fly.
The fly should deflect forward as the sear hits it. It then should come to a stop and force the nose of the sear to jump over it and the half cock notch it is covering.
If the sear jumps over the half cock notch, repeat this test a few more times.
By the way, it isn't uncommon for the nose of the sear to hang up slightly on the fly when the hammer is slowly lowered.
When falling at full speed, the sear should not hang up on the fly.
As a side note: This hanging up on the fly when the hammer is slowly lowered is the reason someone shooting a gun with a fly in the lock should NEVER try to drop the hammer from a full cock to the half cock.
They should always lower the hammer to a position below the half cock and then raise it back up until they year the "click" of the sear entering the notch.
This "hanging up on the fly" with a slowly lowered hammer/cock can happen with all fly equipped locks. Siler, Davis etc. They all can do it.