AS 2 is the type of glass. Per SA standard its glass suitable for side glass, but not windshield. Tempered with at least 70% light transmission. (AS 1 is laminated and must transmit more than 70% light)
M 8,6 is the manufacturers part number
DOT 18 is for PPG Pittsburg Plant
26 71 is for plant number (two digits) and month/year, so July 71.
Penta star and PPG symbol are obvious.
Yes, the shape looks like its from an A-body Dart Sport/Demon/Duster model line