The British Museum used the "MB" (ligate/monogram) on some of their
electrotypes. I strongly suspect that this is one of those. (ie: it is!)
Further, I think the "Auction
House 2018" coin (or did you mean 2012?
is also a copy, given that the original BM coin is . . . still
in the BM! https://www.britishmuseum.org/collection/object/C_RPK-p270A-7-Cau (it also looks like the MB has been smoothed off the 2012 coin)
"MB" as a mark for electrotypes is quite well known, and I am surprised
that an experienced dealer would not immediately recognise it as such.
Even the weight
should have rung alarm bells, with the 2012 sale
indicating a weight
of only 7.09g! (the original is 7.957g)
I'd be seeking a refund, and perhaps an explanation, ASAP if I were you.