Ben's website pretty well sums up the Checiny issue. They are cinderellas & were never used legitimately but there's a bit more to the story. "Poczta Miesjska" translates as "Municipal Post" & it was the Checiny Town council that ordered the stamps at the end of WWI. In the event, by the time the stamps were ready, official Polish stamps had been released & were available. To add to the problem, the council didn't have the money to pay for the stamps, so apparently it was the printers who sold them to the philatelic market. I have seen these for sale on cover with a bogus cancellation, but yours could be a "by favour" cancellation using a re-made Austrian KUK canceller with the top inscription removed.
Full sets of 8 are listed on
ebay for about $20. They are found perf 10, perf 11.5, & imperf.