There is a freeze plug, about 1.5" DIA, just next to the oil plug in the back. But I have a different cylinder head. Mine is a B2000 mechanical head, which fits both engines. Perhaps somebody can chime in on this.
The workshop manual doesn't show any details.
It looks like it would go into the hole above where your plug is sitting in the picture, but I've never had that hole plugged in any of the B2200 engines I have rebuilt.......it may be for a different style of intake manifold that doesn't use (or cover) that hole.......maybe from a different market (or country) since these trucks were sold & shipped all over the world, back in the day.