I'm a Rock owner (though of the Mk.2 "Elite"), but didn't know Max was offering a customized RB300 back then. More recently, he took the 300 and performed some radical surgery on it, installing the damping trough outrigger in place of the Rega headshell, as well as new bearings and other upgrades, and marketing it as the Excalibur Arm. I have found Max to be very responsive to emails and phone calls, so I would contact him to get the wiring info.
But you have another option: There is a machinist in England making two great arms using only the arm tube from the current Rega 300-series arm, making all the other arm parts himself. The company and arm is named Audiomods, and the arms are drop-in replacements for Regas. There is a website for info, should you be interested, or even just curious.