S&W sells their curved butt plates for the 3913 for $3.01/ea (on their website), plus any applicable tax & shipping. Might just send you one if you call and ask, though.
From their Parts List:
Part # 108970000 $3.01 MAGAZINE BUTT PLATE - CURVED
The difference between the Flat & Curved butt plates in clickable thumbnails (shown on a CS9, but they're the same parts used on the 8-rd 3913 mags):
When I got my 3913, I had three mags; one had the curved base, two the flat. I called S&W, told them I'd like to order two of the curved, the guy asked for my address and in two days I had two new base plates delivered, no charge. Never hurts to call and ask...