In this article by Gian-Carlo Rota, you can read:
"I bought a copy of Frederick Riesz' Collected Papers as soon as the big thick heavy oversize volume was published. [...] It was clear that Riesz' publications were few. What is more surprising is that the papers had been published several times. Riesz would publish the first rough version of an idea in some obscure Hungarian journal. A few years later, he would send a series of notes to the French Academy's Comptes Rendus in which the same material was further elaborated. A few more years would pass, and he would publish the definitive paper, either in French or in English."
This made me wonder if nowadays there are some (printed or online) research journals (in English or French) that are somewhat obscure (note: I don't mean "obscure because their target audience is composed by few specialists") and nevertheless publish good works by good mathematicians.