I think you've hit it,some kind of demo or display for salesmen or reps.All three rods,pistons,and bearings are different.That worm gear kind of rings a bell with me too.I've taken something apart with a block and trunnion above the worm like that,but I can't remember what it was on.For some reason I remember the block being all rattly on the worm.
Could be spares for a Chevrolet so it could be rebuilt on the road.
3 cylinder would make it a Yugo