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Here to help - on it! (OP, I am not selling any calipers, no worries)

Great, so now I absolutely must purchase a billet one for my yz250 build.... LoL Go dig around in those black boxes & find me an old one off one of your Yamahas Tony...

The idea with a smaller piston on the leading edge is to create less initial grab/jerk that results in a more progressive modulation/control feel overall. For my slow-ish speed, I experienced this differential piston setup as 2 different personalities in the braking system. A low speed, less grabby mode and a higher grip, more powerful mode. This personality ^^^ is amplified by the billet construction. Make no mistake about it, the stock cast version and this billet version feel markedly different. Your mileage may vary obviously and I found that the better the rider, the more they preferred the billet...