1972 version that starred Charles Bronson. I remember the book, or at least I think I do. I remember the mechanic using chewing gum to make a little cup sitting on top of a gas line. There is a hole in the bottom of the cup where the bare metal of the gas line is exposed. He then filled the cup with acid. Over a period of hours, the acid slowly ate it's way through the metal wall of the gas line, which led to a leak, which filled the apartment with natural gas. The Mechanic then fired a bullet into the apartment that touched off the gas and killed the occupant. Or maybe he just let the pilot light on the stove touch it off, I'm not too sure about that. Anyway, bullets are not good at setting things on fire, at least not the way they show it in the movies. They used the same technique at the end of this movie, and I'm pretty sure it wouldn't work, unless you used a tracer round, which is entirely possible, but they neglected to include that little detail. No matter, I've filled in the blank. There don't seem to be many copies of the book around anymore.
There isn't much to the story, just an assassin, his protege and their victims. If they did their job properly we would never hear about it. It would make a good thriller, but that's not how Hollywood makes movies. Things go wrong and we get lots of gunfights and explosions. Maybe it's better that way. If we were only seeing executions that look like accidents it might be just a little too scary.