Nope,just labeling
That isn't entirely true. Typically... the 12.5 is a newer engine with the bolt in the crankshaft to hold on the flywheel, and the counterweight eccentrics that need to be machined off when using the ARC brass counterweights. The 12 on the other hand, typically... has a nut to hold on the flywheel and removable eccentrics so that you don't need to machine the crank to use the ARC weights.
Those are the only differences that I know. May not be all of the engines either.