this kind of thing is so cool. we know from listings that there are a fair number of klezmer test pressings from the 1910s/1920s that never saw a commercial release. none in this set yet, but I hope some surface eventually. In some cases they were done by people with no other recordings, in other cases a large catalogue but they had some that were deemed not commercially viable and never released.
https://www.library.ucsb.edu/digitized-test-pressings-michael-brooks-collection
Digitized Test Pressings from the Michael Brooks Collection
During the course of a many decades-long career with Columbia Records (now part of Sony Music) Michael Brooks produced numerous reissue projects and was the resident "vault whisperer" who was tasked with tracking down unissued material, alternate takes, and pristine source material from the Columbia and Sony vaults for reissue projects. From original metal parts in the Sony Archives, Brooks would have new test pressings made for evaluation. Michael's record collection and his personal collection of these test pressings was acquired by UCSB in 2019.