The pipe uses a square cut o-ring. The pipe or tubing must be perpendicular to the surface or straight in for it to work
. Bend the tube if you have to being careful not to deform the end (put a snug fitting socket in it or bend it about 2" away from the end with a flat bar). You can get the o-ring from any Volvo
car dealer or most car parts stores. At Volvo the sealing ring is listed in the climate control section for any 440, 540, 122, 144, P1800, 240, 260, 740, 940 series cars. A generic part is available as a "water pump sealing kit" for these same series cars. Volvo made it this way for about 40 years. O-ring gets installed with a bit of liquid soap or rubber friendly grease.
Or you could tap the hole for a 3/8 NPT pipe to hose nipple fitting.