I found this one on eBay, and it appears to have the same castings as my unknown mill engine, and looks to be the same size. I would think that this one on eBay has the correct cylinder and valve chest, whereas mine has those parts made not from castings. So this looks very much like the engine Gil identified as American Model supply in Chicago.
https://www.ebay.com/itm/186109911409?hash=item2b5503 is the eBay listing.
Here is a photo comparison.
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