Hi Daniel,
I think you are right, a water pump with one clack missing, the spring holding the end of its piston against the eccentric, never seen one like it, not sure how well it might work with only a 1/8" bore, 7/32" stroke which along with horizontal clacks wouldn't deliver much water for a twin cylinder 3/4" bore, 1.1/4" stroke engine.
Mystery solved I think. thanks
Take care Tony.