With the breakage problem associated with the original, long, one piece extractor on the original M-39s, ISP sent a number of guns back to S&W and the original one piece long extractor was replaced with a re-designed two piece extractor. The replacement two piece versions works perfectly and should not be confused with extractors on later variants (439, 5904, etc.).

...and the number on the left side of the frame is an ISP inventory number as PJO1425 outlined.
Mine is #236.