Three themes, used by Bruckner for an improvisation at Kremsmünster on August 21, 1884, one day after he composed the “Perger Präludium”, were ublished on page 61 in Anton Bruckner: Bausteine zu seiner Lebensgeschichte (München: R. Piper & Co., 1911). These are not in Bruckner’s work list, but are available through imslp.org via the link below:

http://imslp.org/wiki/3_Themes_for_an_Organ_Improvisation_%28Bruckner,_Anton%29