Yes, you're right about that. It is the Dynamic Brake. The front of the cab was also white back then, as you can see on the photos from 1967. The same probably applies to the side with the long hood. It gets a little bit less clear with the winterization hatch. It's hard to tell if there was one in 1967. Her sister 1721 definitely had it.
http://www.rrpicturearchives.net/showPi ... id=4806668
I have a B&M 1723 here too. Let me see if I'm willing to add these details. That is a pre-production model with a matt lacquered cab.