Here's some Billy the Kid trivia...
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Billy was right-handed.
In the 20th century, it was widely assumed that Billy the Kid was left-handed.
This belief came from the fact that the only known photograph of Bonney, an undated ferrotype, shows him with a Model 1873 Winchester rifle in his right hand and a gun belt with a holster on his left side, where a left-handed person would typically wear a pistol. The belief became so entrenched that in 1958, a biographical film was made about Billy the Kid called The Left Handed Gun starring Paul Newman. Late in the 20th century, it was discovered that the familiar ferrotype was actually a reverse image. This version shows his Model 1873 Winchester with the loading port on the left side. All Model 1873s had the loading port on the right side, proving the image was reversed, and that he was, in fact, wearing his pistol on his right hip. Even though the image has been proven to be reversed, the idea of a left-handed Billy the Kid continues to widely circulate.
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He was known to (or said to) carry a pistol with a "bird's head" grip, which would probably indicate a variation of the SAA. But I also recall that the pistol he carried, whatever it was, wasn't a .45 or .44. Hopefully someone who knows will wander by and clear that up for us.