Your revolver is almost certainly 1952 or 1951. Changes that were made after it was introduced in 1950-
1951 - Slightly larger frame and triggerguard
1952 - Introduction of square butt at serial 21,342
1953 - Elimination of screw in triggerguard, larger grips (four-screw)
1955 - Removed upper sideplate screw at serial 75,000 (three-screw0
1957 - Chief's Special continued as the Model 36
Since we know that SN 21342 was made in 1952, SN 20xxx would have been that year or earlier.
Value? How much did you pay? Well, there ya go. :}
Seriously - we need PICTURES !!! Got box, papers, etc?