Difference between PR3-1,PR3-2, and PR3-3??

[kicker]

Im wondering why b16b head codes are not the same at the end

 

[bipbip216]

I've heard the number represents the casing it was created from in the factory, so no physical differences really. There's also a PR3-4. For the ITR, the best looking heads were chosen and green paint was added behind the distributor before being sent for port and polish! Not sure if the same was done for the b16b.