Hmmm.... Pioneer X
Hi again
From the Ephemeris generator Pioneer X took about 28 hours to reach the SOI, so it was beaten by New Frontiers' 21 hours.
New Frontiers isn't available on the generator yet either *sigh*
I had always thought Pioneer X was launched on a parabolic trajectory, but apparently not. It was launched in March 1972 and swung-by Jupiter in December 1973, which is 1.75 years, so it wasn't parabolic.
Adam