Jim - go to this site and find the third entry block from the bottom labeled: "oder Nummer eines Nummernstempels". In that small entry block, enter the 5-digit Feldpostnummer and click on "search".
http://www.stampsx.com/ratgeber/stempel-datenbank.php
I do not use this myself as I have Kannappin. Someone told me that the Stempel-datenbank has almost all of the FpNs but that there are some missing in the 60000 and above range.
Larry