Hi
Silver plate, not sterling - William Rogers is a well known maker. Refer this link
http://www.925-1000.com/silverplate_R3.html - founded 1865, so dated some time after that. Became part of the International Silver Co in 1898 I understand, which may account for the I S markings. So personally suspect some time from 1898 on? Personally haven't seen that exact mark before for Rogers, but reckon one of our regular experts will. When silver plate makers changed their marks is usually a good way to (roughly) date a piece.
Regards
Waylander