They are what size GMC "Jimmy" Detroit Diesel engine they originally came from.
A 6-71 blower came off of an inline 6 cylinder engine displacing 71cu in per cylinder and an 8-71 came off of an inline 8 cylinder engine displacing 71cu in per cylinder. If you see a V6-71 it would be a V style engine. There are also 53 and 92 series blowers but they really aren't used in performance applications, the 4-71, 6-71 and 8-71 are the most popular. Then there are the monster blowers like 10-71, 12-71 and 14-71.
The Weiand ones are just copies of the originals and never saw life on a "Jimmy".