http://www.worldcat.org/title/missa-pro-defunctis-andrzeja-siewinskiego-na-tle-zbioru-muzykaliow-sandomierskich/oclc/778775801&referer=brief_results

There should be information here - unfortunately there is no MAC plug in for Sun Java to view the document....it appears to be a study of it so it should cite sources. It's been recorded enough times that there must be copies floating somewhere.