I have had a good look at it and it feels like it is pewter.
It could be silver ( All the plaques in my other sets are) But it just doesn't feel right to me.
I gave it a bit of a rub on the back of it ( Don't go crazy its only a plaque

) and it stayed a silvery grey, Much like pewter.
I recon those sellers on
ebay are either misguided or dishonest