That is possible...it could have begun life as a marine engine, but then why is there any stamping on the VIN pad. Also, I've never seen a marine engine whose water jackets were not completely lousy with rust and assorted gack...these are clean.
I had a guy here in town buying 340 from me in the late 70's when his motors blew up in his Drag Line for cleaning ditches. He claimed that they came with 340 in them when he bought them new. It looks like a constuction yellow on that block you have pictured. Could be some kind of industrial if they ever really did put them in construction equipment.:shock: