In all honestly, I never knew Harley had anything to do with racing ever. I was born in 86, and only Harleys I knew growing up were choppers and cruisers. I didn't know Harley used to race AFTER I bought my Buell. Wouldn't surprise me if most of the under 30 crowd assumed the same.
This looks bizarre now, well, it did back then too, but it was not uncommon in the '70s. These guys would enter national enduros or mud runs, whatever. They'd get blown away by European bikes but the big bikes had their own classes.
Somewhere there is a pic of a national enduro champion passing an outlaw biker, full colours and saddlebags, in the mud in a national event.
When you hear "ol' timers" talk about the evils of HD's AMF period, a big part of that was pulling out of every form of competition.