When was Smith Wesson model 37 serial number 21JXXX made?

The Smith & Wesson Model 37, known as the Airweight revolver, was produced starting in 1952. The serial number 21JXXX indicates it was manufactured in the late 1970s to early 1980s, as the "J" prefix was used during that time period. For an exact year, you would typically refer to Smith & Wesson's serial number lookup or production records.