mine doesn't it's 3 numbers one letter and 3 numbers
Sounds like S&W found an older frame in inventory and used it to make your 66-2 so there's no telling when it shipped without a factory letter unless customer service can sully some info on your revolver.
This is not unusual for Smith.They never threw anything away that they could use.
S&W continued using old style s/n's on a limited bases on certain K-frames well after the triple-alpha/four-numeric s/n. The transition was several years.A 66-2 would have an alpha numeric serial number on ....The 66-2 were made from 1982-1986
What about the model 19. Saw a ad today for a 19-3 2.5" that was P&R.