I've designed a few injection molded parts over the years, and getting a precise fit between two parts like that is nearly impossible. After molding, some plastics really like to move around. That's why you'll normally see an exaggerated witness line between two parts, like the curved line between the backstrap and grip on the side. That wide line with no texture makes the varying gap between the molded parts less obvious. I suspect the guys at S&W chose a polymer with extremely good toughness and durability, and traded some dimensional stability and shrinkage to get it.
That said, my small backstrap bugs the hell out of me, visually. There's a sink mark along the back edge that ruins the smooth curve from the top to the bottom.
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