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.
Marcus & Bärbl ... Instructional Videos
Excellent instructional videos from world champions. You will see many references to the dance stylings of Maxie Dorf, Willie Desatoff and other great dancers. Also See: Swing Balboa