I disagree with the tub diverter theory, but feel free to prove me wrong- it could be something so old that I've never seen one (ran my own plumbing and heating business for 15 years). What might help, would be ID's on the pipes attached to it- it reminds me of some older gas furnace parts I've seen, but without dimensions, it would be hard to say.
A mixing valve would be arranged differently- this has all 3 pipes on the same plane, where mixers are usually on 2 planes, to save space inside the wall. The clevis hanger that seems to still be attached reminds me again of boiler/ furnace piping, and would indicate it being rather old, as that style of hanger is hardly found anymore in a supply house. The cutting torch is a very good likelihood, but again, it would depend on the dimensions.
Can you post a picture showing more of it, and the inside diameters of the pipes threaded into it? That would help narrow down the list of suspects.