Jerry,
I think you're right on that score. Various instruments were very well known locally, but would be unlikely to be readily identified elsewhere. I believe Morne lives in South Africa, and despite the fact that I had a great aunt from CapeTown I don't know anything at all about accordion music there.
Slavko Avsenik I can relate to, as there was a Bier Keller culture here in the UK way back in the 70s/80s, and that was the style of music they played in them. However, I never realised Slavko Avsenik was Slovenian until I got into accordion music. We all just thought he was a German with a non-German type name, as a lot of the lyrics to his tunes were in German. There was also an American guy you've probably heard of named Frankie Yankovic, who was of Slovenian descent, and I've heard a few of his records, which were accredited as being in the original Slovenian style.
However, Frankie's style was definitely different from Avsenik's. Maybe his folks were from a different part of Slovenia, or more likely his music was slightly "Americanised".
To further illustrate how the word "legendary" can be very deceptive, I once asked my niece and her husband, who live in Northern Norway, if they knew of any sources where I could buy music by the great Norwegian accordionist Toralf Tollefsen. They both advised me they had never heard of him, yet he only died in 1994. I had heard him play on Norwegian radio in the short time I lived there, and fortunately you can still hear him on You Tube. He is unusual for a Norwegian in that he plays C system CBA instead of B, but he was from Fredrikstad near the Swedish border, where C system is most common. He played a Hagstrom Skandia, but I bet few people on here will have heard of that "famous" accordion. It was the Swedish "Shand Morino" (at least the top of the range Skandia was, as there were several versions).
However, in later life he was also famous for playing a Hohner Morino with that extra three rows of bass buttons that people with short fingers like mine could never hope to reach.