Actually in 1972 and 1973 at Daytona MX, a Harley 350 Sprint was ridden by Harley employee Ken Johnson who was spearheading the development of the rear forked MX bike.

Chilly,

Great piece on the KTM, that bike is bitchen.
But, it is NOT the first four stroke to qualify for a SX main by many years.
But first, ssssshhhhh, don’t tell Moen as he is very sensitive about it. You know how fragile the poor boy is.
Actually, The Rocket, Rex Staten was the first four stroke to make the main at Daytona SX in 1974.
I know, I was there! He was riding a special Honda CR250 Factory bike with a trick big bore XL350 motor shoe-horned into it!
Might have been a Bill Bell motor. I remember it was beautiful, and Wreck It Rox finished fourth or something overall.
There were three motos in SX back then!

Also, in 1972, I believe, kinda foggy ya know, the great Rich Thorwaldsen (RIP!) rode a Rickman BSA to Top American at a Friday night TransAm race at Ascot park, no shit!
I forgot what he did but it was either second or fourth against all the top Euros.
Not an official Supercross but Ascot and Lions were really the start of indoor stadium racing in the USA.

Bitchen web-site!