I got a reply from him
Hi, soda bottles caps are VERY LARGE diameter over 1" as I just measured one for you, original mossberg follower diameter would drop right into a soda cap, check for yourself next time you buy a diet coke (followers are not from soda bottles caps but look of the same concept) - the material used is softer and chemical resistant.) . And if your follower is the same or close diameter as a mossberg it should fit. use a caliper and measure your follower. The outside diameter of this follower is .903 where the spring will thread into, and is tapered down to .883 where it meets the receiver.
I measured a 2 liter soda bottle cap and diameter is 1.203" just in case you were wondering.
Factory mossberg follower measures .905 and tapers down to .899 my follower is narrower as if you use your shotgun in dirty and dusty conditions and "really" use the heck out of it, you will see the failures of stamped steel and aluminum. Take a little dirt...drop it in your magazine tube and see how well your follower glides over that debris. It won't, and will jam halfway up the tube leaving the only option to drive a cleaning rod down the tube if there is a access hole or to remove the tube completely and bang on a rock hoping it would free the shells. I would not want to have to do this if a situation ever arose.
The factory tolerances are tight...too tight for debris in the tube. My shotgun was put though a severe torture test with these followers, reason why there are dings in my tube...and the soft plastic follower surpassed both aluminum and stamped steel 10 fold.
I offer 100% satisfaction on this follower, if I wasn't 100% satisfied with functionality I would not offer the item for sale.
You can make smart remarks about it thats fine. but until you actually go out and get your gun dirty instead of hanging it off your fireplace mantle as a decoration you won't understand the difference between the factory follower flaws versus my designed follower.: