That's a pretty good copy - you can see that it's "just" wrong enough (though only slightly) not to have been taken from a cast made from a genuine coin.
Is that shilling sized? It's unusual in that it's very close to the shilling design, and could be mistaken for one by someone semi-literate who couldn't see the inaccuracy of the legend. Most gaming tokens I've seen are either wildly inaccurate spade guineas with wrong dates, or the 'To Hanover' Cumberland Jack type.
Yes, 79 - 81 was a boom time for coins, then entering a long period of stagnation which only began to end around the time Spink took over Seaby, and Coincraft brought out their catalogue.
If you look at the pictorial examples of different grades in Spink, you can easily come to the conclusion that their grades are very old-fashioned - somewhat lower than the average.
I understand your position perfectly. I'm glad that my own collection was purchased (mostly) before 2000, and has no gold - buying now I would only trust reputable dealers for high value items and would insist on a full and itemised invoice.
My guess (not a legal definition!) is that if the sale is concluded, it's tough luck on the seller - the buyer has bought in good faith and cannot be compelled to return the item.
However, if a buyer asks for the coin at the price offered, the seller is within their rights to say the price is a mistake. The buyer cannot compel the seller to sell an item where a clear and obvious mistake has been made.
Define provenance/pedigree? For example, I'm about to buy an 1806 halfpenny from Michael Gouby. I asked him about provenance and he said all he could remember was that he bought it from Spink at a coin fair. So there's no evidence that it was ever in an 'important collection', but frankly the two names of Spink and Michael Gouby gives me confidence in the coin. Would I have paid extra? Perhaps just a little bit, but it's not that important.
I've not heard of any - the first proofs were the 1971 set. I think those were struck in the early 70s at the London Mint who were too busy before 1971 to bother with proofs.
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