Old Revolver Info Needed

A friend of mine just phoned with a question about old revolvers in the West. He's a writer (and a shooter) and he works very hard to keep accurate with the details in his stories.

I didn't have an answer for him, but I know you all would have the info he needs.

He's looking to identify a revolver with the following characteristics:

1. Readily available prior to 1868
2. Found in the United States in the old West.
3. Uses cartridges rather than cap and ball.
4. Fairly small and light such that a woman could carry it concealed.
5. Something around .36 caliber so it has some stopping power.

Any of you know what he's looking for? Once we identify the basic revolver I can help him track down the details.

Thanks!