Hi Ian, what a little ripper you have there! From my research, that would be most probably the first string trimmer (whipper snipper as we cal them) made by Homelite and is Part Number UT20507 and made in late 1977 or early 1978. Click HERE for the Parts List.
Good score there Ian, and worth preserving!
Please do not PM me asking for support. Please post your questions in the appropriate forums, as the replies it may receive may help all members, not just the individual member. Kindest Regards, Darryl
