M97 riot sg question

[ww2imposter]

I have here a WW2 riot sg with matching #'s. Just out of curiosity I removed the butt stock and found another set of #'s. They do not match the receiver/barrel ser#'s. This sg did go through an arsenal after the war. Has anyone seen this before., 1st two pics are of the "riot sg" in question, the 3rd pic is a M97 trench sg of the same vintage.
http://i33.photobucket.com/albums/d55/boomerangsrd/M1897%20combat%20shotgun%201918/006_zps9e3a4433.jpg
http://i33.photobucket.com/albums/d55/boomerangsrd/M1897%20combat%20shotgun%201918/007_zps647c0af7.jpg
http://i33.photobucket.com/albums/d55/boomerangsrd/M1897%20combat%20shotgun%201918/008_zpsfb36b18c.jpg

[scosgt]

It is not a serial number. If it was it would have been off a gun made in like 1902 or something. Don't worry about it.