Here's a nice example of Duke Ellington's transcription recordings from 1941:

http://www.cduniverse.com/search/xx/music/pid/4743912/a/Take+the+%27A%27+Train%3A+The+Legendary+Blanton-Webster+Transcriptions,+1941.htm

Another hugely important transcription entity was the World Broadcasting Service, for which Ellington, (and others including Miff Mole's dixieland jam band) cut a series of outstanding sides in the early 40s. These were issued to the public years later by the Circle label:

http://www.amazon.com/Ellington-Orchestra-World-Broadcasting-Series/dp/B006QKZPRG