I just replaced a set of verdigris infected hammers on a Steinway O. There is a number that appears to be a date (1937) on the plate but the SN indicates that this is a 1923 piano. On the #1 key there is a manufacturing date stamp of April 17, 1923 that agrees with the catalog date, but what is the 1937 number?
Following on John's comments, in the past if you called with the serial number, Steinway would send a brief history of the piano (date manufactured, sold, any work done later by the factory, etc.) if requested. These days I believe there is a fee for this information.
The date on the side of the key was put there by the fore finisher. They were paid piece work and used signatures or stamps to get credit for completed work. The 1937 on the plate would have been put there when it was refinished at the factory. VR tells us that it was refinished in varnish. The ones that were done in lacquer have LQ in the same location.