Answer: A Whitney revolver, serial # 3110. Source of information: The Whitney Navy Revolver, by Daniel Williams, page 63.
Edit - The sources provided by the players in response to this question strongly support that Stuart was carrying a Whitney revolver, and nothing else, at Yellow Tavern.
However, the answer to the May 7 question here in the trivia game indicated that Stuart carried either a LeMat or a Lefaucheux. There was no indication that he ever carried anything else. Interestingly, nobody mentioned a Whitney in the responses to that question.
I'm not going to mark anybody wrong if they find something here on CivilWarTalk that appears to support their answer. Although evidence has been presented that Stuart was not carrying either a LeMat or a Lefaucheux at Yellow Tavern, I am going to give credit for Lefaucheux as a correct response.
Hoosier