After recently purchasing a model 36 I decided to do a little research on the serial numbers. OP, since your serial number has the J within along with 5 digits, I believe it dates from 1971 to 1972. During these years the J was placed within the serial number along with 5 digits (1971-1972 = 1J1 - 999J99) before it was switched to a J followed by 6 digits in 1973, until 1982 (J******). Eblanche your gun has a pinned barrel. I believe smith and wesson stopped pinning the barrels in 1982, and judging by the serial number it is also in the 71-72 range. Unless there is a total of 6 digits in the serial number which could put it at 1982.
Coincidentally mine also has the J inside the serial number as opposed to the beginning, along with 5 digits. (9*J***) Hope this helps!
Edit- OP, the second reolver you mentioned with no J in the serial number dates somewhere from 1958-1961 although I cant pin point the year.