The production date of Selmer Signet Soloist trumpets can be determined from the serial number. The serial numbers are stamped on the bell of the instrument. The first two digits of the serial number indicate the year of production. For example, a serial number that starts with "05" means the instrument was produced in 2005.
The third and fourth digits of the serial number indicate the month of production. For instance, a serial number that ends in "03" means the instrument was produced in March.
The fifth and sixth digits of the serial number indicate the day of production. For example, a serial number that ends in "15" means the instrument was produced on the 15th day of the month.
So, if the serial number of your Selmer Signet Soloist trumpet is "050315", the instrument was produced on March 15, 2005.