two-bit cowboy's time frame is correct. The floating J numbers were mostly issued in 1971-72. A few appeared at the end of 1970 and a very few shipped beyond 1972 (but were probably assembled in 1972).
Those who have tracked these numbers have indicated that the vast majority shipped in '71-'72, and the numbers as low as 20Jxxx were from 1971.
I have not personally tracked these, but my experience would bear out what others have reported, as reflected in the previous paragraph.
Also, do not confuse the floating J numbers with the serial numbers used in 1982-83. Those always have six numeric digits along with the J, while the floating J numbers have five or fewer digits.