DP means Infantry Division ( Dywizja Piechoty ), so sounds like he was in the 17th Infantry division of the PoznaĆ Army. Not sure what the 72nd Cyclist Company bit is about though. This infantry division was involved in the Battle of the Bzura in September 1939:
en.wikipedia.org/wiki/Battle_of_the_Bzura
en.wikipedia.org/wiki/17th_Infantry_Division_(Poland)