I have heard the same, that there is a service manual for the 8C twenty. If you can find one it will work for you because the 22 and 8C 20 are the same basic tractor as the PL 15. The 22 has bigger engine and heavier duty finals.
If there is a book for the L 20 and PL 20 or 25 it will do you no good, they are a different animal.
Maybe if you ask at ACMOC.org someone will know what books are out there.