About 35 years ago I repaired a lock that had been Perverted from flint to percussion and it had a sliding safety that engaged both the tumbler and the hammer. In the back of the hammer was a slot that locked into the hammer at the same time the internal part engaged the tumbler. When the safety was slid back,it released the tumbler and the part that engaged the hammer moved back into a circular area that allowed the hammer to rotate to full cock and then fire the gun. This is about as close to a positive safety as will ever be found on an operational gun lock. The "safety" of today's locks,mine included are designed to withstand the torque of the mainspring and hopefully become a noticeable impairment to firing the lock from "half cock". The old term "going off half cocked" originated with the defective or forced to fire lock.
Right after I joined this forum,I noticed a posting about someone testing a double set trigger against a half cocked lock. That kind of trigger test can be done with the hammer at rest without endangering a sometimes fragile
part of the lock.
Getting back to sliding safeties,later high quality English caplocks sometimes had one forward of the hammer that had the slot and semi circle on the back of the hammer that engaged the projecting part that was out of sight behind the hammer. In England,the "Patent Mania" and work with gunlocks turned up some novel ideas such as Manton's gravity activated safeties that allowed loading at full cock while the self priming locks had the frizzens closed. The idea was to be able to level the gun and the gravity operated safties would fall away and the gun at least in theory would be fireable.
Hope this doesn't muddy the waters too much.
Bob Roller