Hi, Clogdance, & welcome from a "new guy". My trusty "Striking Impressions" book refers to Type I & Type II mintmarks on 1979 & 1981 proof coinage, specifically the "S". The Type I was produced with a worn punch, resulting in a "barely recognizable blob". The Type II resulted when a new punch was introduced, resulting in a much more defined mintmark. Basically, the processes repeated itself in 1981. Hope that helps.