Another long shot to try: When I replaced the steering box on my prior YM186D I googled a casting # I saw on the side of the box and found mention of it in a forum where someone was restoring a Datsun 240Z. The trademark logo on my box matched his. So Yanmar must have been using a generic steering box supplied to Japanese car manufacturers as well. Going down that rabbithole might take a lot of investigation. I ended up buying a rebuilt steering box from Hoye.
PM sent, re a scrapyard that said he is now out of business. He would have had a steering box. If you get a useful reply from him, who he sold his inventory to or something, please post it here.