It's all gone a bit quiet on this thread.
Hey, chaps, when I suggested measuring with the plane assembled rather than with the parts dissembled on the bench, I wasn't intending to needle or 'get at' anybody. It was a genuine suggestion. If I've upset or offended anyone, I apologise unreservedly. That wasn't the intent at all.
The reason I suggested checking IN the plane was this. The two-piececap-iron applies pressure to the cutting iron (pressure coming from the lever cap) differently to most one-piece capirons.
With a one-piece, the capiron beds at two places, one right up close to the cutting edge, and one near the top of the frog casting. The pressure at the cutting edge tends to bend the blade back and slightly down, pivotting it at the heel of the cutting iron's bevel or at the base of the frog casting. That tends to make the middle part of the cutting iron lift a bit from the frog casting. Thus, when a cut is applied, it can act like a flat spring, 'fluttering', and when conditions are right, vibrating and causing chatter.
The two-piece capiron beds in the same two places as the one piece, but critically, also at the joint between the two pieces. The lever-cap pressure is applied to the cap-iron nose-piece BEHIND the cutting iron bevel or frog bottom end, so some of it's pressure is applied to the top of the cap-iron nose-piece (at the cap-iron joint). Together with the lever-cap pressure at the top of the frog pressing the top-piece down (and this piece is in close contact with the blade for it's full length), this presses the blade into contact with the frog casting at the cap-iron joint, thus stopping it acting like a long, flat spring, and stiffening things up considerably compared to the one-piece cap-iron.
The Bailey patent one-piece is intended to transmit the lever-cap pressure in the same way as the two-piece, trapping the blade against the frog casting. For some reason unknown, most one-piece cap-irons don't quite conform to the Bailey patent, though.
Thus, the two-piece capiron gives a better-performing plane than the same plane with the same blade and a non-Bailey one-piece cap-iron, because the whole assembly uses lever-cap pressure better to give a stiffer assembly.
You can demonstrate this with a six-inch steel rule on the bench. Clean off a patch of bench with a nice, well-defined edge, and place the ruler at right angles to it, hanging-hole end pointing towards the back of the bench, and about 10mm or so overhanging the front edge, which represents the base of the frog casting. Place a finger firmly on the hanging-hole end, representing the top of the lever cap pressing against the top of the frog. Now put a bit of finger pressure at the tip of the overhanging end of the ruler, representing the toe-end of the cap-iron bearing close to the cutting edge. You'll notice that the rule bends, pivotting about the bench edge. and popping up clear of the bench at about the 2 1/2 inch mark. Now with the finger pressure still on the hanging-hole end, apply another finger at about the 1 1/2 inch mark, representing the pressure of the two-piece capiron at it's joint. Now press down again at the 'cutting edge'. You'll find the same pressure causes much less downward deflection, and than the middle of the rule, being trapped, doesn't pop up. Everything is much stiffer.
I agree with Douglas - the Stay-set and the Clifton two-piece are worthy of praise.