On a broadcast with Bird and Diz from Birdland 1950/51(?), Symphony Sid Torin addresses Diz as "Birks" and is told by Diz to "Watch your language!"
And doesn't Sid reply "I said Birks, not berk"?
Can it be that Cockney rhyming slang (berk for 'c' word that rhymes with Berkshire Hunt) had crossed the Atlantic?