It is a 50M. It is also exactly the same as some Vito branded horns. As the story goes, these were being manufactured for Leblanc (Vito) by the Art Best company. Conn bought out Best along with the factory and unfinished stock. Apparently a good number of unfinished horns had already been stamped with those low serial numbers (The Vito branded ones also have low 4 digit numbers) but must not have been engraved yet. They were finished with the Conn shooting star pattern. Conn continued producing the 50M, but at some point they started putting their own serial numbers on once the stock from Best was depleted. I theorize that they also changed lacquer at this time bc the 4 digit ones all seem to age to that dark brown color whereas later ones with the 6 digit numbers seem to stay light.