My guess is Billy is the tall one in back, and Pat is front and center.
IIRC in one of the movies, Pat and Billy were cordial to each other, so this photo from before the arrest and killing would seem to make sense. I don’t recall the names, but it seemed like back then a lot of the lawmen may have had questionable histories.
My folks retired to Las Cruces, over 30-years ago. Their home, which is now my sister’s, is on the East Mesa, about a mile or two from where Pat Garrett was murdered.
Garrett was not a very popular fellow, around those parts and Billy was a hero. Back then, Las Cruces didn’t exist and the town was Mesilla.
Old Mesilla is an interesting place.