Muzak was one

Muzak was one of many transcription services for which musicians--many
of them now regarded as legendary jazz artists--regularly cut
recordings that weren't peddled to the public in stores. The only time
anyone heard these sides (which often exceeded the 3.5 minute
limitations of the ten inch 78 rpm record) was when they were aired
over radio waves as filler. Transcription recordings by Duke
Ellington, Claude Thornhill and dozens more have been reissued and
often represent some of the best material we have from that period
(30s&  40s). I believe this was largely due to the fact that the A&R
directors usually associated with commercial recording sessions were
absent or "defanged" for the transcription sessions, which weren't
geared towards selling units in stores. Interesting, huh. Check this

Jazz, dance band and pop artists whose transcription recordings have
been reissued by the amazing Hep Jazz label include

Boyd Raeburn, Gene Krupa, Sam Donahue, the Pied Pipers, Stan Kenton,
Billy Butterfield and Artie Shaw.

This JZCL compilation (a double CD not to be confused with a similarly
packaged sampler released by Stash records) contains what I believe
are all of Fats Waller's Muzak recordings from 1935 and 1939. The best
moments (in addition to certain group jams from '39) are the piano
solos, which underline perhaps the most important and least recognized
aspect of Thomas Waller--his subtlety. Not to downplay the majesty of
full-throttle swing! For rowdier current try "Nagasaki" and "The Moon
is Low".