Where is the serial number on a mossberg 185 k-a?

Your shotgun was made between 1950 and 1955. The law did not require serial numbers on rifles and shotguns until 1968. Simply put, your Mossberg has never HAD a serial number. Yes, it is legal to possess. It is NOT legal to remove a serial number, but if the gun never had one, that's OK. 1950-1955