I used a small allen wrench to grab the locking pin through the slot on the side of the magazine to slide the spring towards the follower to remove tension (and the enclosed detent pin from the hole in the floor plate) to slide it out. I cranked the tapered (closed) ends of the floor plates in a bench vise and the gap closed tightly.
The base plates are now sealed in the bottom of the magazine when assembled and no issues loading or ejecting the new magazines. No issues firing.
I also bought spare TSW4566 Magazines and these were fine.
Smith & Wesson should have better standards for whomever makes their Model 41 magazines. I edited this photo to show where the gap was...
