I also thought Center Industries when I saw they were blank. Adventure Line was purchased by Parsons Precision Products which was purchased by Center Industries. Maybe there is a connection there.
I also went through my Preban magazine collection and specifically looked at the followers. The followers in these unknown magazines were almost identical to Adventure Line magazines. The mold mark was in the exact same location. I also looked at Colt, DPMS, FN Cooper, General Stampings and Okay. Each of these had unique mold mark locations on the bottom of the follower. Interestingly, all the Unknown magazines had the mold marks 1, 2, 6 and 7. I assumed I would find 1-7 in a somewhat even distribution but I didn’t.
I’ve also attached a comparative picture of excellent condition Adventure Line magazines to the unknown magazines. The horizontal stamping imperfections are extremely similar.
I have reasonable confidence these magazines were made by Adventure Line, Parsons Precision Products or Center Industries. Since Center Industries is still around, do you think anyone might have documents related to the company history and old contracts?