hi Ross - in view of the op's comments in reply No. 9 - it would seem to be silver rather than pewter. From experience when cleaning silver, the colour produced is most definitely black - whereas pewter doesn't give the same effect (I've just tried it).
From my limited knowledge of silver/plate, I thought that most of Europe included marks/Nos. of some description on silver - so in the complete absence of any marks might this indicate the Far East??