I have an SW 40 C SN PAM 1203
The model 40C (.40 semi auto) was made from 1995 to 1998.
I don't know much about the Sigma series so I can't offer much help.
To get a closer date you'd need to call S&W. They'll give you the DOB free over the phone.
Smith & Wesson
Mon-Fri 8:00AM-8:00PM Eastern Time