"I believe it is older than 1968"
The Stevens shotguns I've seen were serial numbered up until WWII. After WWII they didn't, until required to by the GCA of '68. One expert on shotgunworld says they were all serial numbered prior to WWII.
Starting in '48 iirc, they used the 'number & letter in a circle' code. Well, the code was the letter, but there has to be number in the circle too.
Savage/Stevens/Springfield made a variety of guns and models in the years around WWII and seemed to combine parts at random some years.
After looking at the pics, it looks like an old Stevens shotgun. I used to feel inadequate when trying to figure out which was which, but not anymore because they should have marked the model clearly. Everybody gets confused by them. For instance, the ones like yours but marked 5100 on the side of the receiver are supposedly a Model 515 - the 5100 refers to the model number of the receiver design, etc.
P.S. - My uncle has my grandfather's Stevens SxS hammer gun from the early '20s and I still don't know what model it is. It's worn smooth with use and one of the hammers was welded together after it broke in half and I don't mean it was welded by a talented gun smith. My grandfather grew apples commercially his whole life and probably did it himself along with the other repairs.