The schematics for that era are all contained in one book which you can buy from Victory Glass Company.
# 60-1087 (R-381) Schematic diagrams for all Seeburg 20 play models made from 1938 through 1949, 95 pages, just $29.50
Individual service, parts, operation manuals are available (no schematics) depending on the model name like "Major", "Envoy", "Cadet", etc. Just talk to the folks at Victory Glass and they'll sell you the correct manuals you need.