My seriall # is J137XX. I went through the old issues of "Gun Digest" that I have ( not a complete set ), and this is what I learned. The Lightning was introduced in 1984. It was available with either a tapered or heavy barrel, both of which were 18". In 1987 they introduced the AMT Lightning Small Game Hunter, with a 22" Target barrel, and regular stock ( not folding, like the previous ones ). Price then was $269.
I then had no more issues 'till 1998. Somewhere before then is when the SGH II came out. In the 1998 issue they show the AMT 22 Target Model, introduced in '96. Price was $549. I remember those, they were "full-race", with trick looking stocks ( I think still plastic, but a high comb, pronounced pistol grip), and I think a Jewell ( or similar ) trigger. They were the "last hurrah" for AMT.
By the way, my SMG II has a stud mounted in rear of reciever, facing toward butt, to act as a second mounting point to the stock. Does your earlier version have this ( stock has to be removed to see it ). It sort of cantilevers into a recess ( hole ) in the stock.
Also, FYI, in the 1990's AMT made a "Magnum Hunter", a .22 mag, but it shares nothing with Ruger, has a traditional magazine, etc.
Hope this helps a bit.
Mark