For the last 2 days I have searched every corner of the Internet looking for an answer about the correct, original master cylinder used on a 1968 GS400. I'm trying to do a manual drum to power disc conversion, but I want to use all factory correct parts. I have MOST of the components in hand EXCEPT for the master cylinder. Does anyone in Buickland know what the correct MC casting number and 2-letter stamped code is for a 1968 with power (4 piston) disc brakes? Does anyone have a survivor car or documentation that could solve the mystery? I know many will work, but trying to be as original as possible. THANKS!