What about the Bicentennial quarter. Would you call that a stopping and restarting point of the Washington quarters?
Here is how I see it...
The 1976 Quarter has the same obverse (save for the dual date) as any other 1932 to 1998 Quarter (save for the continual and easily overlooked changes that all coin designs have each year). The bicentennial was a special, two year (1975-1976) modification to the (standard) Washington Quarter and is part of that regular series.
The Statehood, Territorial, DC, and ATB
quarters were designed to be different and I believe intended to be distinct from the original Washington Quarter. They have a significantly different obverse: a smaller head with rearranged (Liberty, In God We Trust), added (United States of America, Quarter Dollar), and removed (the date) legends. Whether you break these down into one (Commeerative Quarters), two (Statehood/DC/Territorial and ATB
), or three (Statehood, DC/Territorial, and ATB
) series is up to you.
Personally, I see the Washington Quarter as having three distinct series.