I'm the original purchaser of the coin, I funded it shortly after receiving it with mined Bitcoin (why I said they are virgin coin). The first 5 coins were graded by the creator before being sold. I must've had the certificate, but if not I would have funded using the first bits. I recently moved across the country so it's possible the certificate is still packed away somewhere. Seems strange that I wouldn't have kept it with the coin if I had it, but anything's possible.
It does have a very cool backstory being funded like that and I don't discredit you being the original owner as I've misplaced some of these COA's myself when they get separated for grading like that.
These coins are very nice and I'd venture to say if it was graded back in the day, it could regrade better than MS67 now since the grades have been skewed higher in the past years. That definitely helps the premium since the coin could be a MS68/69+ upon regrading today.
Best of luck if you decide to search for the COA and I'll be a bidder for sure if the address is verified!