I would guess that it is a 252707. The number is stamped into the shroud, most likely just above the spark plug. They are not on a tag or decal, just stamped into the metal. In my two decades in the business I only saw one engine that didn't have numbers. That one was on a prototype White mower that we bought from the booth at the OPE expo. The engine may well have been a prototype as well. Wipe the dirt off of yours, the numbers are there.