Hi -
If you have to remove the grille to mount the speakers, it's post 1969; if you have to remove the back to mount the speakers, it's 1969 or earlier.
As far as the 2X15 cabinets, I rather prefer the front-loaded design for sound. But for appearance and mojo the rear-loaded cabs are beyond cool. Either sounds fabulous, even with other than JBL's. To this day, the old Sunn 2X15 cabs are the very best.
Ric