The main distinguishing factor is that one set of rulebooks says "second edition" on the inside cover and the other says "third edition". I don't believe the rules changed materially between the two. Of course the set itself contained different things over the years. Originally Holmes came with Monster & Treasure assortment and Dungeon Geomorphs. Then B1 In Search of the Unknown for a while and finally B2 Keep on the Borderlands. The switch to B2 seems to have occured around the time the "third edition" booklets were printed, but some "second edition" sets contained B2 instead of B1 toward the end of their production time. All "third edition" sets contained B2.