Many moons ago, GM wondered, “is it Dumbarton St. or Dumbarton Ave.”? After searching through maps from two centuries, GM reluctantly concluded that it was probably Dumbarton St.
Well, final evidence has come his way. The blog of the DC Master Address Repository recently featured the above map. It was the official map documenting the switch in street names after Georgetown merged with the city of Washington in 1871. It was officially ordered and approved by the Office of the Commissioners of DC. So there’s really no more definitive map.
So look at the close up, and the question is settled: it was street before the merger, and it was street after.
Oh well, GM still likes the sound of avenue better…
(h/t to @IMGoph)
The Georgetown Metropolitan 
Oh great! Now every resident on Dumbarton has to change their stationary.
Thanks for the mention of my blog. If there are other similiar street name issues I can research.
glad to pass along fascinating maps!