Disk or drum? I'm assuming you're referring to disk. If so, the original style master was the low profile unit with bleeder screws, 1 1/8" bore, shallow piston bore for the booster rod.
Classic says the 67-68' master cylinder casting # ends in 346 and for 69' it ends in 309.
I can find master cylinders ending in 309 and I believe the description below is correct. Does anyone know where to find one ending in 346?
And, does a shallow vs deep hole in the master cylinder make a difference for which power booster will work?
Brand new not rebuilt. Has the original casting number of 5468309 like original. Licensed GM Restoration Part. Cast Iron with Bleeders 1-1/8" Bore; Deep Hole; 9/16" & 1/2" Ports on Left Side with Bleeder Valves; Exact Copy of Original. DOT 3 brake fluid only.