It is a 4" which somebody has round butted. All the 4 " and 6 1/2" 24-3s were made on the square butt frame. Only the 3" Lew Horton Special was made with a round butt frame.
Since only 2,625 4" guns were made, somebody has ruined a nice collectable.
Interesting side note, a poster over at The High Road remembers ordering two of the 4" 24-3 guns in 1984, and they came to him with RB frames... I'd be curious to know the serial number on this one...