1972, actually (with the exception of the sixth playing, taped in January 1973).
Of those six playings, three of the contestants won only the first prize. One contestant won both prizes with one second left on the clock. And two contestants won both prizes as the clock hit zero.
And the seventh time they played Clock Game on the Dennis James version (the first time they played it without the $1,000 bonus), the contestant won both prizes with nine seconds remaining.