The ofna buggy names/numbers are kind of just scrapped together to sound like the Kyosho/Muggy's, but if you follow the Kyosho and Mugen numbers they actually give a fair amount of info.
Kyosho being the simpler of the two simply uses version numbers - MP5, MP6, and if a version is a completely new design/platform, they make it the next number and add 0.5 to it. Because the MP5 to MP6 was just a few parts upgrades, it was left as is. The MP7.5 is obviously a new platform.
Mugen on the other hand releases a line of buggies and then changes the letters at the end to signify which is which. Example - MBX4 was the original, MBX4-RR was the next, slightly hopped up and slightly changed version, and the MBX4-XR was the all out no-holds-barred version.
Chances are, we will see an MBX5-RR or MBX5-XR sooner or later too. Obviously they will have more hopups, and new design changes to keep up with the competition (I'm thinking that will happen as soon as the new Kyosho design gets released.... although that won't be for at least another year methinks.)
all manufacturers use different methods, but usually you can work it out pretty easily.
hope that explains
uDi