Hi hammerback. I don't think anyone knows for sure how many Model 21s were made for sure. Many of them were scrapped during their production process. Plus, serial numbers jumped all over the place. The serial numbers went up to (W) 32,000+ and I have seen a few of the older guns with higher serial numbers. People say that there were probably less than 30,000 of them made. Just taking a guess, I would say yours was probably made around the late 50s to early 60s. A 20 gauge with three inch chambers is kind of rare. Enjoy your gun.
