How fascinating! They are absolutely the same marks as those shown http://www.ascasonline.org/ARTICOLOSETTEM210.html
, making it likely that the coasters (if that's what they're called) and the decanter labels were meant for each other, even sold as a set.
What's interesting to me is that they came from a family line that had quite a bit of very fine silver, some of which is now in the Met and other museums. And I'd always assumed they'd been in the same house since ~1906. FWIW, they held decanters that are genuinely from the 1700s. They seem quite old, and the two have slight differences, which makes them appear to be hand made.
Perhaps there was chaff amid the wheat! What a mystery!
Many thanks Steve,