The difference between a standard size 71-series blower and the later "small diameter" blower is barely perceptible, and would make little difference in performance on a street engine. However, you don't want to mix these rotors and cases. Here's a quote from the book, Street Supercharging:
In approximately 1978, GMC introduced a new line of 71-series blowers, labeled simply "4-71" and "6-71," which have smaller diameter rotors than previous 4-71's and 6-71's. (The reason for this reduction was to reduce horsepower required to drive the blower, to reduce fuel consumption.) The difference in rotor size is hardly perceptible, even with a pair of these blowers side by side. The exterior of the cases, and the end plates, gears, etc., are the same. However, the large (early) rotors will not fit in the small (late) case; on the other hand, the small rotors will fit the in large case, but will give clearances that would make the blower nearly ineffective. Either size of 4-71 or 6-71 could be used for an automotive performance blower, with aftermarket end plates, drives, manifolds, etc. as long as the rotors match the case, and are clearanced properly.
In other words, the reason it's so hard to tell externally is because the walls of the later small rotor case are thicker.
The 6V-71 has an oddball case requiring a less common style intake. Supposedly Dyer's makes (or made - past tense) components for the V-71 series. I'd stay away from them.
