A pianos serial number will usually be found stamped on its soundboard in figures about 2 cm high.
Serial numbers are usually between four and seven digits long.
A number stamped on the top of the side of an upright piano is probably a dealer's stock number. A number cast into the frame is almost certainly not a serial number. This Bluthner Grand's serial number is nestling below the strings just behind the dampers. This Bechstein Upright has its serial number stamped near the top of the frame. The serial number of this Waldstein is stamped onto the soundboard below the left action bolt.