According to the SCSW, SNs ending with TCB was the end run of 1988 and in 1989 they started with TED serial number prefixes. I think the 3913 began its run in 1989 so I'm guessing yours was in that first run (89). Some others here will probably add some insight into the numbering system S&W used. (As confusing as it is)
Hope this helps.