Sterling silver items are always marked with the number 925 somewhere on the piece. They may also bear the word “Sterling” stamped on the item. For fine silver, look for the imprinted letters FS or the numbers 99.9 stamped on the piece.
To distinguish silver plated items from other types of silver, look for markings such as EP, Silver on Copper, or EPNS (electroplated nickel silver) on the item. 

The box and tag have probably been added later as the holder didn't know what it was.

As for the date, I am going by Sheffields date stamps. But if it's rugby union then as you say my date seems too early.

Otherwise I get 1916 according to London stamps.