I have one of those too, but no manual. They were recalled in 1977 for electric cord problems by CPSC (Consumer Product Saftey Comm.). Amazon has a picture of a loaded string reel here: www.amazon.com/Black-Decker-Trimmer-Replacement-RD-300/dp/B00004R9TP
but they are not in stock. Is a picture worth a thousand words? Maybee... good luck!