Difference in gear boxes

Okay I might look like an idiot asking this but, lol I don't really care.

So how many different kinds of gear boxes are there and what are their differences?

Here is a good article about all gear box types: http://www.acmeairsoft.com/showthread.php?t=5820

From my understanding (low low low) each different version is made to fit into other styles of guns that the other ones won't fit into. Otherwise I don't know poo

I believe there are Eight (Sorry, Seven) versions....
V1: FAMAS
V2: MP5, SCAR(modified), M16's M4's, all AR's "M" series
V3: AK, AUG, and G3
V4: I believe this is the SAW. There is another technical term for 249 mechboxes, PGC.
V5: Uzi(TM), which has a full sized mechbox.
V6: P90 and Thompson
V7 M14


EDIT:
AHA! I was right!! there are eight, that TM Burst GB....