There is an NRPS from 12.15.73 of Glendale Train with Marmaduke and what appears to be
this guitar on youtube...
That's it for sure. I loved that Glendale Train too!
So that puts his guitar at least sometime in '73.
Also supporting that; in the older pictures Rob posted, you can clearly see there is a brass plate between the pickups; this would have been where the batteries went in an early instrument, and the humcanceller was glued inside. That feature went away at some point in '74, and battery boxes were located in the center of the back, like were used to. In later pictures, we can see the humcanceller has been epoxied (?) in place between the pickups. Dawson must have used the batteries often... note how he has a 1/4" cable connected in almost every shot? Wonder if it was wired mono so both pickups were heard?
Look at that cool handcut logo on brass, screwed right to the front of the headstock! How'd they do that?