72 was the changeover year. I guess the plants just used up their supplies and some cars had them, some didnt. I think 68 was the first year for the plastic ones.
68 Valiant 408 69 Charger 318 69 Cougar 351W 70 Torino GT 351C 71 Country Squire 351W 71 Road Runner 440+6 71 Satellite sedan 318 73 Duster 318 73 Challenger 383 77 Grand Prix 455 83 Malibu 9C1 383 84 Delta 88 403 87 Grand National 96 Ram 360 09 Crown Vic 10 Challenger R/T M6 15 Challenger Hellcat M6