david weaver
I think Bill's response was appropriate (or maybe it was someone other than bill), that describing how it's held in place doesn't account for their being a difference.

Every stanley cap iron is made to mate with the iron, too, they just don't have to vary because there isn't variation in the cutting iron. The mechanism for holding the second iron in place varies because of the cutting iron, not because the second iron itself dictates that it should be different.

I haven't read odates book in a while, and I don't remember whether or not he says anything about second irons, but the fact that a japanese plane can be operated without them doesn't suggest that they're not intended to fold a shaving. A japanese plane certainly doesn't need a second iron for stability because of the abutment, and the fact that the plane can work fine without them only suggests more directly that their presence at all has something to do with deflecting a shaving.