I think it’s a ’40 or later motor Kiekhaefer designed and built for Montgomery-Ward. The earlier Thor built motors look completely different. Here’s my ’36 or ’37 Thor built Sea-King with the stamped steel gearcase which is bolted together and angled on the front, instead of rounded. The sheave on it looks the same as the eBay one though.
I had a couple of those aluminum leg Mercury Sea Kings with the boxy tanks, awhile back. Sold ’em for $50 apiece. They are not real common but they’re out there.