Hopefully Barry will be able to remember the exact part number he used.
If he can't remember- my guess, based only on the 1-1/4" bore diameter and general shape, is that it might be a Wagner (or Raybestos) MC-4355. This is not certain but may help others in researching the answer. A posting on the G503 forum
http://forums.g503.com/viewtopic.php?t=222244 claims the 4355 master is correct for the G506 Chevrolet as well as 3/4 and 1 ton pickups of the period - apparently
not CMP correct but period Chevrolet.