Upon touchdown, the World War II era tailwheel-equipped airplane experienced an uncontrollable swerve as the right main landing gear began to collapse. As the right main gear exited the edge of the runway, the airplane rocked forward onto its nose and the propeller and left wing tip impacted the ground. The airplane rocked back and came to rest partially off the runway in an upright and right wing low position. An examination of the landing gear retraction system revealed that the landing gear handle detent was contaminated with paint. Subsequently, the landing gear handle did not stay in the detent which resulted in the right landing gear downlock pin to partially retract, followed by the partial retraction of the right main landing gear.