I suppose this would be the grand daddy of them all (well, except for the crazy stuff that Mike Byers has for sale). I bought this off-metal strike earlier this year. It is a 1977 $100 gold coin struck on a nickel 50c planchet (it is magnetic). The planchet sizes are identical - but I suspect there was some mint workers at play, at the Royal Canadian Mint in 1977. This is the coin, as it was intended to be struck, proof struck, 22K gold. It presents well inside a double slab from PCGS. I guess it should not be too hard to find a gold 1977 $100 coin, in a PCGS MS-68 holder, and upgrade the set to a matching pair...
Very cool SPP. You don't see to many gold coins or proofs with off metal errors. So, was it more expensive than the gold lol. And is that your double slab or are you just thinking of getting a gold and having both double slabbed like that one?