The 70 Charger Registry did a fair amount of research on this...specifically for 70 Chargers which were all built at the St. Louis Assembly Plant. We found that the Month was spelled out on the decal from the beginning of the 70 production run (August 1969) through to cars built on September 12th 1969 (SEPTEMBER 1969)...on the 15th of September the format changed to the numeric format of 9 - 1969, 10 - 1969, 3 - 1970 etc etc....however we have documented cars were the format was 1 / 70 and 2 / 1970 styles...these are not common.

1970 Charger R/T Videos http://www.youtube.com/user/70CHARGERREGISTRY?feature=mhum#p/u