From: The First Gen Camaro Assembly Process by John Hinckley

The chassis then passed into the fluids area, where the power steering system was evacuated and filled automatically, and the brake system was evacuated-and-filled automatically through the master cylinder - the bleeders on the master cylinder and at each wheel were never touched.

From my observations of original cars and research in the original 60's parts books --

- Any car with power brakes which the booster orientation resulted in the Master Cylinder NOT being level / horizontal received a MC with bleeder(s); and

- Any car that had a MC that WAS level / horizontal received one without bleeder(s). That could be manual brakes, or it could be power brakes with no booster tilt.

Despite there not being any need to bleed them on the assembly line, the issue was future service. If the MC were removed and serviced and reinstalled, or if fluid got low enough to get air in the top of the system before refilling, then there can be an air bubble just above the line attachment caused by the tilt of the MC. The bleeder(s) allow elimination of that bubble at the MC, rather than having to try to push it all the way down the brake line to get it out. A level MC did not create that issue.

As for service replacement MC's, after some period of time of servicing both styles of a given MC, they typically stopped selling the version without bleeder(s) and only offered the version with bleeder(s). That may not be true for all of them (such as a MC that was never used in a tilted application). But if you trace the part number supercessions through the parts books, that was usually the case.