The trouble with this type of thing is that you really never know unless the guy has the receipts with the engine number on them.
I wonder how he turns the engine every month since there is not flywheel and no front pulley. Maybe a prybar on the flywheel bolts?
I agree with Don that, IF you get it for the right price (which for an undocumented engine should be less the the opening bid IMO), you'd be best to buy a head set and tear off the head, timing cover and oil pan for inspection before running the thing.