It is hard to say when the Charleston progressed into the Lindy Hop, but credit is often given to George "Shorty" Snowden for making the transition. The only known film record is his performance in the 1929 Film "After Seben" in which Shorty is performing some Charleston moves when he breaks open into a one-hand lead which gave rise to a new type of dance - Lindy Hop. Some of the original moves also eventually migrated into Balboa. Arthur Murray put out a good training film in 1937 presenting this form of Shag some of which also migrated into Balboa.
After Seben (1929 Movie Clip) with George "Shorty" Snowden. Watch and you will see a few steps that were eventually used in Balboa.
Tiny Balboa - Teacher's demo (very good)
Hal Takier - Dance Clips 1917-2012
Marcus & Bärbl ... Instructional Videos
Peter Loggins & Others
Balboa Part 8 (final)
More History Links
All Balboa Weekend - Cleveland, OH
Historic Balboa Pavilion - Newport California
H. Leon Raper