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Ah! OK... I'm working/hoping to get adequate performance from the OEM but am wanting alternatives in case I can't get sufficient performance. FWIW, my CB400 is going to be my normal rider, not a weekend or "special event" bike. Therefore, as much as I would like to retain the OEM gear/appearance, It's less important than being able to STOP.
A CBR 600 master cylinder will be for a double disc and not for a 400/4 single disc.
I'm going with the CB1300 MC Mike, it has a mirror mount and a front brake light switch if you ever need to remove the one on the bottom yoke. Look identical to the CBR600 one although I can't see any mirror mount on Roos pic.
The CBR600 master is most likely a half inch or 12,7 mm, which is fine for a single 38mm piston caliper. On all Honda masters the piston diameter is cast on the outside, most often facing the handlebars.