It's that word 'Polowa'. The root is 'Pole' which means 'field', hence '
Polska' - the country of fields. However, if you put a diacritical accent through the 'l' then you get the word 'half', hence the problem in translation.
So yes, "Poczta Polowa Doplata." means 'Polish Fieldpost Postage Due'
Except that there's no such genuine beast. What I suspect you have are these which are the bogus cinderella machinations of one Captain Alexander Stocki

Captain Stocki escaped to France after the fall of Poland in 1939 where he perforated "W P" ("Wojsko Polskie")on current French stamps, posted some covers bearing these perfins, & convinced Yvert that they were genuine Polish Fieldpost stamps. They show up from time to time, Cherrystone sold 30 of them for $350 a while back.
Stocki "W P" perfins


He then showed up with the Polish Army in Exile, eventually based in Edinburgh, where he joined the Polish Philatelic Society from which he was later thrown out for misuse of their cachet.
His main claim to fame in this area is his production of so-called Polish Fieldpost stamps & seals such as the Postage Due labels illustrated above. He even muddied the field by producing a booklet after the war called 'Polish Post Seven Years War' in which the French & British labels are listed with the justification that they were official emissions.
THE tome for this era of Polish philately is 'Polish Exile Mail in Great Britain 1939-49' by George K. Kay & George Negus still available through J. Barefoot.
A recent history of Stocki has recently been published, don't have it yet but I have a birthday coming up so may treat myself, info is here -
http://www.royreader.plus.com/To recap, these are unofficial private labels but certainly collectible cinderellas.