Howdy
Sometimes a photo helps. Here is a photo of the mechanism of a #1 1/2 Tip Up, but all the Tip Ups were similar. Notice the hump at the top of the hammer. The hump interacted with the bolt in two distinctly different ways. The bolt is the part sticking out of the frame towards the hammer. There is a spring under the bolt keeping it pulled down and engaged with the locking notches on the cylinder most of the time. A portion of the bolt protrudes through the frame and engages the locking slots on the cylinder. The bolt is actually partially split in half, more about that in a moment.
When the hammer is cocked, the radius at the rear of the hump on top of the hammer rocks the bolt up a tiny bit, pulling the locking tab out of the cylinder notches, allowing the cylinder to rotate. As soon as the hammer clears the bolt, the spring slaps the bolt back down, so it can pop into the locking notches on the cylinder, locking the cylinder.
When the hammer falls, the sharp pointed leading edge of the hump separates the two rear halves of the bolt, so the spring will continue to keep bolt down with the cylinder locked in battery.
It is actually a very clever and simple system.
If the hammer will not cock without raising the bolt, then something is wrong. Perhaps a burr on the hammer is preventing the bolt from rising. Perhaps the radius at the rear of the hump is worn. Perhaps there is dirt or something preventing the bolt from rising. Perhaps the spring is broken or not working properly.
P.S., no, none of these had any sort of safety device built into them.
