I’ve been looking for this one on and off since about 2004 with no luck. Turns out its attached to the Rockville ice rink which is a bit off of a minor road in an industrial park and is impossible to see until you’re right on top of it. It’s a Kullman and according to Richard Gutman’s The American Diner Then and Now, it’s from 1997.

50 Southlawn Ct. Rockville MD.