All E20's have a cast front axle as well, but I have seen the cast front axle on E15's too.
From Lee in Tennessee: I hate to be disagreeable but...My E-20 has a steel front axle and my E-15 has a cast iron front axle. (Of course the original could have been replaced before I got it!)
I've been told that the steel is better because it won't break like the cast iron one supposedly will. But I've had this one with the cast iron axle for over 20 years and have run it into trees and what not and it hasn't broken yet. (I have bent the tie rod a time or two, tho)