7/28/08-In doing several Open Reel extractions I came across one that simply said "Glenn Miller Airchecks". I searched and hope this answers the question-
Before tape and digital, they were 16 inch one of a kind transcription disks ordered by either the bandleader or sponsor of a radio program
for later review. A few of those airchecks survived
I also checked the songs on the reel to see if any known commercially released collection matched the play list from the reel. I listened to a comparison of the Intro on the Moonlight Serenade -Paradise Restaurant Show dated 12-23-1938 to the Intro of the same show on the Aircheck Reel.
I was convinced that this unique collection was different.
Both sound bytes are below. | 
Advs of Philip Marlowe
