Alf,
this is the skinny on that number according to the boys at S&Wforums - the famous (albeit that hardly anyone knows about it) 'Floating J':
In 1950, when the factory introduced the Chief Special, they gave it a new number series and started it with 1and by 1969 reached 786544. In 1969, to comply with the Gun Control Act of 1968, they had to give it a unique number so they added a letter J prefix plus five numbers.
Somewhere near the end of 1970 they ran out of five-digit combinations at J99999 so some clever person at the factory came up with the idea let's start the "floating J" series and in 1971 began with 1J1 going to 1J9999 . . . still retaining the J plus 5 numerical digits. Then, when they ran out again at 1J9999, they went to 2J1 to 2J9999, then 3J1 to 3J9999, then . . . well you get the idea.
Ah, but wait . . . there's more to the story. When they reached 999J99, the factory stayed with the J-prefix but used six numbers beginning with J100000. That lasted until sometime in 1983 when they adopted the three-alpha plus four-numerical format.
And now you know . . . the rest of the story.
Russ