Just remember if it is real and the only way people got them were if it was stolen from the mint then arguably if could be government property (Think 1964 peace dollar) on the other hand if even 1 or 2 were sold or given away to friends or private collectors, if it is real then could be a fantastic score! Best of luck to you getting it authenticated, hope its real and worth heaps for ya (One member recently scored a proof florin valued over 50k so Extremely rare coins sold by those who do not know or realize can happen and if its real you may just have him beat!) Not to sure who the Australian Expert on sovereigns is but my guess is it might be worth a trip to one of the major auction houses!
One of the places I went to thought he might know who the contributor who added the two 77s sovs to the big index book was and took a scan of it and is sending it to him. It's a real ray of hope that a couple actually exist so the mint that year had everything they needed to make them and it is at least possible and I wouldn't have to do a big historical search to prove that they exist at all. I've also been referred to noble coins in Sydney. Ideally I'd like to find a picture of one if the couple that are known to exist to compare. I wonder if Sydney mint would have a historian or researcher there who could help.
Jewellers copies were made to showcase the expertise of the jeweller, hence they are the same purity and of just a fine quality (at their best). To further prove the quality and to set themselves apart, choosing a non-minted date and or mint would be the evidence its not simply a real one. You cant 100% discount the possibility the dies were switched (this should be a shield obverse instead its st george / or vice versa) but i think it unlikely. 1for1
Dies wouldn't necessarily have to be swapped out as it seems a small run of them was actually made. There are 2 known surviving examples according to the book a coin dealer I saw today had (forgot to ask which one but it looked like a phone book). Still likely to be a jewelers copy or straight forgery as you say but it is hopeful information as they did make some though most were destroyed there are a couple of survivors in a similar fashion to the 1920s.
that would be Krause which has the following 1876M 2,124,000 BV 245 750 1,250 1877S Rare 2 1877M 1,487,000 BV 240 700 1,200
That's it. Thanks. I'd be curious to know who he tight contributed knowledge of those 2 and where they are and who owns them. I'd kill for a picture to compare notes.
I have several sovereigns in similar cardboard coin holder marked by reputable coin dealers mixing up the S and M mintmarks. So what we haven't seen here is a close up picture on your coin mintmark area. I think you have an 1877M.
If you have a look at the video it's definitely s mint mark, this photo isn't great but shows it as well. Source:
Obviously I should see it under my magnifier, but from the picture it looks good. You should try now to get an official option and recognition if the coin is really genuine. One way to do this is to grade the coin with one of the two leading US grading company. If this coin not genuine they will not grade it. Good luck anyway !.