That is a Nock lock from Buck Ridge Flintlocks. The "R" inside the triangle is their trademark. The fly is built into the tumbler. It is held in place by a pin that goes through the tumbler. The fly is not meant to be removed. These are really good locks. I've used a bunch of them. Everyone that I have built a rifle for using this lock has nothing but good things to say about it. The trigger is also the Cain style trigger from Buck Ridge. According to their website, the trigger must be set before you can cock the lock. However, with some careful filing of the trigger bar, you can get it to cock without setting the trigger first.