It's not Diesel 35 or RD6 according to my Parts Number books.
Looks to fit a 23C, 67D, or 66D series engine, which could be a 3306 or D333C industrial engine,
Yes, that's half a two piece exhaust manifold for a 66D D333C Industrial Engine part #'s 7L0382 and 7L0383, per 66D parts manual.