The Sep. 1948 edition of the National Bulletin includes an article on the 1948 meeting held at Camp Twin Echo. While the program booklet and the patch issued for the event call it "Area F," the article specifically refers to it as the "Area III-b Fellowship meeting. This is because this event occurred immediately after the integration into the 12 BSA Regions was announced.
The article describes how the "delegates from Anicus Lodge hurried from the  National Meeting [at Indiana University] directly to Camp Twin Echo of their East Boroughs Council of Wilkinsburg, Pa., to take up their duties as hosts for the Area III-b Fellowship meeting slated for September 4-5-6. This meeting had a registration of 365 Scouts and Scouters from 12 lodges in this area..." All subsequent 3-B events have been numbered from 1948.
Trivia: Camp Twin Echo was also the site of the 1940 National Meeting.