GDay Omega, found this for ya,
I had K on my 73 XJ6 Jag rims & changed them to a J rim so the tyre doesn't roll of the rim as easy during hard cornering or out of control moments
J, JJ, K, JK, B, P AND D : TYRE BEAD PROFILES / RIM CONTOUR DESIGNATIONS.
The letter that typically sits between the rim width and diameter figures stamped on the wheel, and indicates the physical shape of the wheel where the tyre bead meets it. In the cross-section on the left you can see the area highlighted in red.
Like so many topics, the answer as to which letter represents which profile is a long and complicated one. Common wisdom has it that the letter represents the shape. ie. "J" means the bead profile is the shape of the letter "J". Not so, although "J" is the most common profile identifier. 4x4 vehicles often have "JJ" wheels. Jaguar vehicles (especially older ones) have "K" profile wheels. Some of the very old VW Beetles had "P" and "B" profile wheels.