These early dates are quite a bit more difficult to track -- the reference we typically use to estimate ship dates (The Standard Catalog of Smith & Wesson, Fourth Edition by Supica & Nahas) doesn't show anything other than a wide range before the 100xxx numbers which began in 1970.
The Model 39 started life around 1955, the 39-2 began in 1970 I believe. The best bet outside of requesting a factory letter is to dig around online and try to spot other folks who have requested the letter with a serial number near yours.
That's what I have done and my best guess on my 31xxx Model 39 is 1964. Thus... it seems somewhat likely that yours is between 1964 and 1970.
