One of the interesting things about the Brittish Royal Mint is that it sells not just new sets, as does the US Mint, but it also sells trasures of anitiquity. I guess the US Mint at one time sold CC Morgans that they were sitting on for nearly a century, but this is a little different. I am not sure how they source these coins, but they are not extra inventory due to historical happenchance. See this for example: https://www.royalmint.com/our-coins/ranges/historic-coins/monarch/james-i/ I noticed that the Israeli mint similarly sells ancient coins and sets from time to time, making them both straigh coin dealers.
Yes, I have tried twice to respond - the RM have a relationship with Sovereign Rarities. RM sources recent releases through the latter and the latter "funnels" older classic coins to them. RM may get coins from other sources as well; these are NOT sourced from the RM collections per se.
