A while back, I was thinking hammer extension and said hammer spur. I got told about it right away. The point I was attempting to make got lost, because we were worried so much about nomenclature. The hammer spur is the part of the hammer that you put your thumb on to pull it back. Usually it it heavily cross hatched or has a crisscross pattern on it. A hammer extension goes on the hammer spur and sticks out to the side so you can do the same thing with clearance for your thumb to the side of the hammer spur. It is splitting hairs, in my opinion, but people will call you on it; at least they called me on it.
With a Handi Rifle, once you cock it, the action locks up so you can not open it. As mentioned, there is no half cock, so you need to lower the hammer carefully to get it into a safe condition. You just have to make sure you have control of the hammer from the time you pull the trigger to release the hammer until in is in the rest position with the trigger not being pulled after the hammer is released.
Good Luck and Good Shooting