It seems to me that the units with the external coils are smaller in diameter than the all-in-wonders. The cap with the coil in it wouldn't fit (too big).
Of course, I've never touched one with an external coil, so I can't say for certain. I'm just saying that if you get a cap, you might just want to go and get a whole dizzy from a boneyard.
I think that we're all on the same page. I'm just on a different paragraph. (not sure if I'm ahead or behind, though...)
