much different fin density and inside shape

 

That much is clear. But whats stranger is better = lower rated for about the same size. And with both garrett and precision if you keep everything the same but increase the flow length the hp rating goes up, so that rating cant just be the volume of air it can flow.

 

It must be (sort of) arbitrary then, maybe based on some combination of the heat shedding ability of the core combined with pressure loss at certain flow rates?

 

Speaking of Spearco, I have this older Spearco of unknown orgins sitting in my basement. Looks like it is offset/staggered.

 

Interesting. .Your Spearco also has the name embossed.