Try asking people at the Dulles airport, or better yet, at
The Steven F. Udvar-Hazy Center near Washington Dulles International Airport is the companion facility to the Museum on the National Mall.
National Air and Space Museum Steven F. Udvar-Hazy Center
If you contact them, poke around until you get a knowledgeable airplane buff. Chances are the first person to answer the phone or e-mail won't know and won't know who to ask.
It only flew that route once a day, not more, in its later years. I have no idea about earlier.