This is a link to a club in england that has a copy of the manual.
The Furness British Motorcycle Club located in the North West of England town of Barrow in Furness.
To borrow any of the titles please contact Tony Gee on club nights.
(i'd say if you approach Tony with a few bucks he'd probably be willing to photocopy or scan the manual for you)
heres the library page:
(#52 on the page)
Technical Handbook - TRIUMPH 500cc TRW Engine Nos 14401N to 25764NA
good luck! i helped some triumph newbies with one here in Vancouver BC recently. the TRW's have a very distinct note to the exhaust, you'll love it. Flatheads Forever!