The release of the AASHO database reveals:
Not only was the DC Beltway requested to be I-68 (which I was aware), the Baltimore Beltway was requested to be I-72...(Apr 1958)
The next iteration (July 1958) has 495 and 695 respectively, except southeast of I-95 south of the city they had it as 195.
The original designation for what is now 270 between 270 Spur and 495 was 170
I-295 was I-195
Go here - https://grmservices.grmims.com/vsearch/portal/public/na4/aashto/default and search for MD and Correspondence to find these...
Wasn't there also to have been an Interstate 63 within Washington D.C.?