Can anyone help me to identify these marks please?

[corporez]

Hi Everyone, I have just purchased something which I can only describe as a large butter dish. Can anyone please help me to identify the marks? Thanks very much for your help. Not sure whether its silver, silver plated ??

The marks are R S B then a Crown and a Swallow (Bird)

Any help will be much appreciated. Thanks. Keri

[corporez]

Here is an image with the marks. Thanks.