Go to the current S&W website Parts List:
http://www.smith-wesson.com/wcsstore...ist_Retail.pdf
Compare the following parts & part numbers between the CS9S:
108970000 MAGAZINE BUTT PLATE - CURVED
239720000 MAGAZINE BUTT PLATE CATCH ASSY
200800000 MAGAZINE FOLLOWER
201650000 MAGAZINE SPRING
To that of the 3913:
108970000 MAGAZINE BUTT PLATE - CURVED
239720000 MAGAZINE BUTT PLATE CATCH ASSY
200800000 MAGAZINE FOLLOWER
201650000 MAGAZINE SPRING
The difference between the CS9 & 3913 magazines? The length of the mag bodies.
The curved butt plates fit on both.
The earlier 3913TSW (original model) had a different butt plate.
The regular 3913 & variants with the same length grip also have an optional flat butt plate.
200810000 MAGAZINE BUTT PLATE - FLAT
Here's some pictures of the 3 different butt plates on a mix of CS9 & 3913 mags (original TSW wide/flat, regular flat & curved):
Reg flat & original TSW wide/flat:
Curved, flat & orig TSW wide/flat:
CS9 with curved butt plate on mags: