You either lost 6th, or you have a 175 or larger model. I bought a ´79 DT125 straight from a dealer in August of 1979 and owned it for 14 years before selling it to a friend. It has 6 gears.
Another friend of mine had the 175 model (same year) and his had 5 gears... plus all the magazine articles back then stated that the ones _larger_ than 125 had 5 speed gear boxes. The 125´s were the only DT´s that had 6.