Probably the early 70s.. I know from research that a coin like that one was priced at $4 in 1967. It's just not inflation, there are also more collectors of these coins these days thanks to the internet, so demand and prices will be stronger.
I'm glad to see it made it to you safe and sound. Interesting that the paper envelope has travelled with the coin so long! Was there anything written on the back of it?
No, there was nothing on the reverse. While it is interesting that paper envelope still exists, I would not use it ever again to store the coin. I have no idea if it has sulfur or acids, so it is quite suspect. I may end up eventually cutting it up and turning the written part into a tag by gluing it onto a piece of card paper. I don't know.
Yea, I'm not sure when archival paper coin envelopes came into wide-spread use. Do you know if the coin had been stored in it? If so, I see no ill effects. And I agree with Pishpash - don't cut it up.
I'm sure someone out there is buying archival and acid free holders to store the coins they just treated with Deller's Darkener. I suspect some of our favorite toned coins got that way from being stored in a less than archival manner. Among the coins with Provenience I value in my collection a a few from the 'Bavarian Collection' sold by NFA in 1993. The collection included many very, very ordinary coins stored in envelopes an with tags made from scrap paper. Several had parts of postage stamps or other evidence of the source of the paper. The romantics among us like to think that the collection was assembled by a WWI disabled Vet (German) whose coins may have been low end but whose numismatic status was excellent. I doubt he knew the term 'archival'. http://www.forumancientcoins.com/dougsmith/feac16.html
And that's why I don't really lose any sleep worrying about the paper envelopes I store my coins in. 100 years ago no one did either and the coins survived, in a lot of cases for the better.