I'm pretty sure it can't be the 4013 because I think they were staggered 9 or 10 round mags and they were in production far longer than 3-4 years.
S&W did make the CS40, which was a subcompact .40 cal for a while. It was part of the "Chiefs' Special" line. I'm pretty certain that was a single stack 6 round mag. Only a 3 inch barrel though, vicious recoil with hotter rounds from what I remember reading.
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