which explains that serial numbers are usually 5 or 6 digits long, so 1157 is not it, although most pianos have several numbers inside, and we can't always say what they all mean, and it seems too big to be a model number. If you slide out the music desk, you may find a number painted onto the iron frame below it, or imprinted in the woodwork inside the top of the piano. It may be to the left of the strings. Most of the removable parts (such as the music desk) may be imprinted with the last 3 digits of the serial number, and this helps you to recognise the real number when you find it. Obviously you won't want to get involved in removing the legs, but if you have no luck, try looking underneath the piano. My Broadwood has the number in large figures under the keyboard. You could unscrew the pedal unit, but if you crawl underneath to look instead, check first that there is absolutely NO movement whatsoever in the leg joints.
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If you find old references or links on this site to pianogen.org, they should refer to pianohistory.info