the only original white series was the 1982 20th Anniv series amps & the JCM 900 wasn't out then.
the cabs should have been JCM 800 cabs labeled Anniv Series w/ Celestion 65's.
http://www.olywa.net/blame/mar/20thAnnv.htm
Dr. Tube used to have a site w/ a great history of Marshall amps but I can't find it now but here's their schematic site-
http://www.drtube.com/en/library/schema ... ll-schemas