Just to expand, on the Model A's the number is stamped just behind the first body mount hole on the left side frame rail. So, you'd need the body off to see it. As mentioned, rust may have obscured it. Also, the first digit is actually the letter A, followed by up to 7 digits, then the star (actual star not asterisk). These were hand stamped so aren't necessary in a perfect straight line. The intention was to match the serial number on the engine, but it didn't always happen that way, and over time engine swaps made that meaningless as well.