There is a book titled "The History Of Marshall" which is available that you may find helpful. It was written in the early-mid 1990's by Michael Doyle. There's a lot of handy Marshall info there. You may also want to visit
www.schematicheaven.com which I've heard is a good source for schematics and also consult with the Marshall distributor for your country as they may have some archives available to help you out with.
Mike 777 Haug