As far as we know, Archbishop Ján Maria Michał Kowalski (1871–1942), the longtime leader of the Polish (Old) Catholic Mariavite Church claimed much spiritual power, even a kind of Messiah-status, but that he never explicitly claim the papacy. Still, bishops in the Mariavite core group, at least from the mid-1920s, asserted that the Roman pontiff was not the true pope anymore, that the Holy See had moved from Rome to the Mariavite centre in Plock, and that Kowalski was the long-awaited ‘Slavic Pope’, that Polish nationalist authors wrote about: a liberator and a benevolent religious leader.
The founder of the Mariavites was Sister Feliksa Maria Franciszka Kozlowska (1862–1921), often called Little Mother (Mateczka). She claimed to receive divine revelations–‘understandings’–from 1893 onwards, and the interpretation of them played a significant role in the development of the Mariavite doctrine, both before and after her death. Posthumously many followers believed Little Mother to be divine, and Archbishop Kowalski had an almost sacred status, even during his life. Claiming ‘understandings’, too, he introduced drastic doctrinal changes throughout the 1920s. Still, Kowalski’s autocratic rule and the unorthodox doctrinal development led to a schism in 1935, when only a small minority of the faithful remained with him.
A preliminary version of my text on Kowalski and the Mariavite papacy is found here