SectorNine, in terms of American engines, the number after 'R' (for Radial) in this case was the cubic inch displacement of the engine. Since it was (and still is) nigh impossible to accidentally design 2 engines with the same displacement, the displacement became the standard number of classification.
For the Merlin, or V-1650 to the US, there was only that one kind. There were V-1650-3's/7's etc as there were Merlin 45's/63's etc but the engine is the same. This is just like R-2800-8/10/18W's... all the same engine with different mods. So really, if the digits in between the dashes changes, it's a different engine altogether.