For those of you who aren’t real familiar with 1908-1915 Proof gold coins - in addition to their beauty, original examples of each year offer their own distinct color/appearance/texture. My personal favorite is 1911, which typically displays - as I have seen it described - “crystalline, nearly diamond-like facets”.
Numismatists who have examined enough original survivors of each year can determine the date of the coin, just by looking at the reverse. Sadly, so many of these coins have been messed with over the years, that a lot of them have lost their distinctive appearance. That makes Mr. Koessl’s set all the more special.