What is this thing? (Electrical)

[ezneily]

looks like a split charge diode, try googling that and see if it matches what you have.
They are made so you can charge two batteries from one alternator and as an option charge three batteries from one alternator. These split carge diode devices did have the drawback of dropping about 1v unless the very expensive type was fitted wich drops about 0.2v.

Not very efficient but simple

example Sterling Power ProSplit D 70A Split Charge Diode D70A2