Sorry you are wrong. The xr69 was always a 1000cc bike and was first raced at Daytona in 1980. It was supplied by the factory following Paul Dunstalls rather unsuccessful Yoshimura powered attempt in 1979. Even Sheene could only manage a 4th place on the Dunstall.
Prior to 1980 they were Yoshimura prepped GS1000 framed superbikes. These did start off as GS750S before the GS1000 was introduced.