I have found many original pre war bikes with copper fuel and oil lines. Jd's, Vl's, 45's, Knucks, and UL models. As far as I know copper was the only material used until the early '40's. I have heard of 1941's with aluminum lines but never found one. Gas tank crossover lines continued to be copper into the Panhead era. Does anyone have a pre war machine with the original copper lines still installed? They were originally nickeled.
While searching for any factory documentation of the Bruce found this document. Have a look at item number 9.