And the number of blades only suggests the right spinner shape.

Spits changed a bunch when the version changed because of the engine. The later versions entire fronts changed and the spinner shape was only a part of it.

The earlier Mks used a shorter, less streamlined spinner. Their "pudgy" look is easy to spot on the mks used in the desert. And most of the early mks are more easily identified by their exhausts. They have 3 each side.

If you plan to do a Spit that had 3 collector pipes each side, you'll want a shorter spinner. You can make a generalization that as the power went up, the spinner got longer and more pointed. Problem is that there doesn't seem to be a guaranteed pattern. Some of the earlier Photo Recons had 3 pipes and pointy spinners.

I'm assembling a H9 60 size Spit, and they don't even mention the spinner size, much less which one to buy. But it takes a 3-1/2" and somebody (maybe Horizon?) suggests using the P51 parabolic spinner. At least that's the only one I've found that fits the width spec.