The single shot rifle was actually a Farquahrson. If you look at one and think, "It looks like Ruger borrowed heavily from this for its single shots", you'd be right.
In the book by Patterson, the incident went down as a misfire in one barrel of a double. He hadn't had much experience with doubles, and forgot he had another shot. The lion ran off just about the time it occurred to him.
There was no howdah pistol in the real story.
In the real story, the angry-mob-about-to-kill-him scene wasn't because of the problem with the lions. It was because he had been paying masons by the month to quarry blocks, at a higher rate than the other labor, so (naturally) everybody was a mason. The work wasn't going well, so he started paying per block (per properly cut block, that is), and the "masons" weren't happy. He went down to the quarry one day to see what the problem was, and that's where the mob scene took place.
The failure of the boxcar lion trap happened exactly as in the movie - someone shot through one of the chains on the gate, which caused it to fall.
Tended on occasion to jump into the basket with you. This pistol was to dissuade him from having parts of you for lunch. 
