Actually 74 -75 where the first ( Yamaha) I remember it well as was just getting heavily in the sport seriously. I had Suzuki RMs in 76-77 then jumped ship to Yamaha monoshock. Qualified for the first time in 78 on a Yamaha to the Pontiac MI supper cross. Raced and qualified for several yrs after also. Spent many yrs ( on numerous Yamahas) in the sport retired from racing at age 26-27 riding my last 5yrs in open A on Honda CR500s best times of my life.. for kicks shortly there after I tried and ran GNCC ATV ( open 4stroke) racing and took a bronze metal at the nationals, just missing a silver metal. That was a trip! There , I just gave up how old I’m getting....When did dirt bikes first go to a rear mono shock?
I remember going from 2 shocks to one around 1981?