If im not mistaken there is a length restriction on shells for water fowling just like the gauge restriction .

I have seen old vintage catalogs that showed 8 and 10 ga shells in just about every length up to 4" . It was probably when they used all brass shells like you said . Seems like back then you could pretty much get anything you wanted custom ordered .

I bet the 12 ga 4" would have a horribly long shot string .

back then- longer shells were meant for longer wad columns to improve the pattern, not to stuff more lead in there - that is a modern phenomenon

There is no shell length restriction only nothing larger than a 10gauge holding more than 3 shells. I never understood the need for the 3.5" shell. I've actually gone away from the 3" shells and found the 2.75" loadings do very nicely. Your mileage may vary.

There used to be a law perhaps it was a state law ? shells couldnt be over 3.5".
Now though on the DNR website I couldnt find any regulations for gauge restrictions . Like for turkeys it used to be nothing smaller than 20 ga well for any large game hunting . Now none of those rules seem to be on their website .

12 ga 3.5" shells have a terrible shot string , there was a video of a fellow testing out different 12 ga 3.5" loads and the shot string was about 12ft long IIRC but I dont remember what range their moving target was .

I do remember reading about using more wading in longer cases to cushion the impact from firing to lessen the deformation of lead shot .

I have found several of the Fox Gun Co., Balto., Md., U.S.A. guns which were only made in 1898 and 1899 to have 12-gauge 3 1/4 inch chambers.

Shot string is way less important than ammunition manufacturers make it sound. I'll let someone else figure out how far a goose flies while a 12 foot shot string arrives. A bird flying at anything less than a 90 degree angle from the shooter can forget about a long shot string saving his life, especially when the proper ammunition is selected. Read Bob Brister, Shotgunning The Art and the Science, chapter 23. He tells it all.