From CRG site:
To distinguish an early (pre-suffux[before 21-OCT-68]) M22 from the other Muncies when the transmission is on a car, note that all M22s had the lower, forward, passenger-side maincase boss drilled and tapped for a magnetic drain plug. This wasn't done (by the factory) for M20/M21 until 1970. While this boss could be drilled for a plug on a M20 after it left the factory - if the boss is undrilled, then the case definitely was not part of a M22.
Also decoding:
http://www.camaros.org/drivetrain.sh...missionNumbers
For the additional drain they refer to looks like it would be where the arrow is.