I just can't leave NGK's alone....

What is the difference between NGK's BCPxxx vs. BKxxx ? From their chart (goto http://ngk.com/sparkplug411.asp and click on "Manufacturer's Numbering System NGK" at the bottom of the page):

B = 14mm thread diameter
C = Hex Size 5/8"
P = Projected Insulator Type
K = Hex Size 5/8" Projected Tip (ISO)

Does that mean BCP = BK ? Well, the BK5ES doesn't exist in their catalog, so I'm guessing that the designation between BCP vs. BK is based on whim at NGK....