Wine coaster

I picked this up today for £1. I just assumed it was silver plate at first but after polishing it and discovering a mark, I think it might well be solid silver. Not much weight to it, but always nice to find some of it. What do you reckon?

 

Is the mark IS?
If so, I'm thinking the coaster is from one of the many "International Silver" consortium of companies, many of which made silverplate... but that was an American thing, I think.
But WDIK!

 

Could well be true. If so you have truly broke my heart. :0(

 

I'm over it already

 

It doesn't look like an 'I' to me. It looks rounded at the bottom (or top if the mark is upside down...)

 

I was looking at letter date codes but then realised that there are no other marks such as crowns, anchors etc so god knows. It looks like there is some kind of shield shape indent that surrounds it though.

 

An actual close-up pic of the mark might be useful. The magnification of the current photos probably shows a distorted image of the actual mark. The S doesn't even look like an S in the "close up" photo I posted.