Looks like a development of their previous unit, which I think didn't have that "bubble".
I remember 3 B&M buildings in Chatsworth when I worked for them '70-'73, one being the Sportscoach plant.
One had the corporate offices upstairs, and I think they'd left Van Nuys altogether by that time.
That might help date it, or look inside the shifter.

The main hiring requirement seemed to be being a racer.
With Kent Fuller, KS Pittman, and an endless parade of other notable racers either working there or passing through, lots of fun memories.