There's something about Tveitt that you might not know... even though this "great fire" occured, Tveitt left us a sort of diary where he inbluded the concert programs from ALL of the concerts that he had. This 'diary' shows us that he performed this 29th piano sonata quite a lot. BUT... there is NO document of him playing ANY of the previous piano sonatas in concert.
so, now the question is if all of that music was actually lost or if it simply was never written....clever guy, Tveitt...