According to CRG , 1969 JL8 cars were only assembled between February 1969 to May 1969, so the trim tag on the firewall would need to show the assembly date between
02A up to up 05E.
There was also a difference in the master cylinders, quote from CRG
“1969 J52 disc brakes used the US-stamped master cylinder. 1969 JL8 four wheel disc brakes used a similar master cylinder, though the stamped code is unconfirmed at this time. The only difference between the master cylinders was the J52 master cylinder had a check valve in the outlet for the rear drum brakes, whereas the JL8 master cylinder did not have any check valves.
Check CRG website out as it has plenty of info to keep you occupied!
If a real JL8 he hit the jackpot with only 206 made in 69. Good luck.
Is that your 70 Chevelle? I want one!