No letter prefix may be O.K.
The serial number is stamped on the butt, and they don't necessarily have a letter prefix. No. 623,xxx is enough - it was made during the late 1960's.
There is some confusion because they were numbered originally in the same serial number series with the model 36 Chief Special. Then an "R" prefix serial number series was started in 1969. The stocks that have a large diamond around the screw were discontinued during 1968, and if your revolver has this feature it could help date your gun.
Serial numbers: It is advisable to not submit a full serial number on a public forum unless there is a good reason to do so. However all you need to leave out is the last two numbers, such as #123,4xx as that may allow one to better date it without compromising your security.