Yep, the first couple years of production of the 3913TSW used a 7-round magazine, like the CS9 pistol. (The same period 4513TSW uses 6-round magazines, instead of the newest 7-round magazines)
The 3913TSW my friend uses has the shorter, cutaway grip frame that uses the 7-round magazines. He also wanted a rubber grip, but we couldn't find one at the time. I finally had to take the regular Hogue grip for the 3913 and cut it off to match the dimensions of the cutaway grip frame. The combination of soft rubber and harder plastic makes the job less than easy, but it's possible ... and once I figured out how to use a combination of moto-tool cut-off wheels, Exacto-knife, a file, and some very fine emery cloth ... it actually turned out to look acceptable, too.
I had to do the same thing for a few of the early 4513TSW's that used the 6-round magazine, instead of the newest 7-round magazine, for the same reasons.
It doesn't take long to ruin a $19 retail, or $13 wholesale, set of rubber grips, though ... and in retrospect I'm not sure I'd bother if I had to do it again ...