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DirtyErnie wrote:State has one of these at school, SN# in the 63xx range, three valve, silver. Anyone care to guess at the year of manufacture?

Sometime just before the turn of the century. See:

http://www.musictrader.com/york.html

Take a careful look at the bell--does it say "York and Sons" or "York and Son"? The singular offspring signals an instrument made between 1887-1898.