You'll have a hard time cloning them without taking one apart since they dont use stanard bobbins, and the shape and size of these pickups obviously contributes alot, i think theres a chance the blades are actually the magnets aswell, although i could be wrong since i've never opened one up.
His L450 (now L600) humbuckers used polepeice magnets and it has been said similar sounds to the L500 can be achieved with them.
here are some of the specs
Clean = 2.8 Henry
Rhythm = 4.8 Henry
Lead = 6.8 Henry
XL (extra lead?) = 9.2 Henry
Total Depth (incl. rails/pole pieces): ~0.85"/~21.5mm
Bill Lawrence L-500/L-510/L-6xx humbucker = 1.350"/~34mm Width x 2.80"/~71mm Length
(~3.35"/~85mm long inclusive of the screw tags)
Cant seem to find the resistence right now, but an ebay seller claims the reading got from an L-500 lead to be 14.97k which sounds about right. Although at one point this changed (lowered i think by a few 100 ohm) to compensate for different magnets used, when alnico was used instead of ceramic due to availability.
you should be able to find out more here