Paul,
Its 41-602. BTW, here's a really cool site for plug x-reference.
http://www.sparkplug-crossreference.com/ Converts the numbers for you.
The 41-602 crosses to an NGK 6 or 7. I tried the 7 and found them a bit too cold for my motor. The 6 works very well. Another alternative is the Autolite 3923. I use the NGKs because of their heat range bracketing. I MAY get some KR at torque peak (4500-5000rpm)but it usually lasts for 1-2 frames(17frames/sec) of the datalog. I had WOT timing up around 34d at 7000rpm on the dyno and got no detonation w them. In any case I would rather foul a plug than hole a piston. Give those shot.