A lot of the pre-war Polish stamps had HUGE margins that you could make seemingly-imperf stamps from. All stamps that are available genuinely imperf are listed in the Polish Fischer catalogue (the standard for Polish stamps). They are usually defaced with a red crayon line & are unused, having been 'liberated' by the Nazis from the Polish Printing Works during WWII & released onto the philatelic market.
The 1923 500mark, Scott 169 is available on thin & medium thickness papers, in 3 distinct shades & the copy illustrated above is well within the shade variations, it's not faded. It is available imperf. horiz. mint, as well as imperf with defacing red crayon mint only.
There are over 20 perforation varieties in this set. As war reparations after WWI, the Poles got some of the old Austrian perforators. Gibbon's Stamp Monthly quotes a post-war stamp printer as saying that the Czechs got the better ones, the Poles got the rest.
The imperf Scott #298 25gr Belweder Palace is a cut-out from a pre-printed envelope. 25gr was the standard domestic letter rate & there were 18 million envelopes printed between 1937-'38
The stamp design is identical to the regularly issued postage stamp Type I measuring 28.4 x 21.3 mm. ( the postage stamp was later reprinted in a larger size, 28.6 x 22.2 mm)
Here's my copy

1937 26 July. Luck to Poznan. Pre-Stamped Envelope. Domestic standard letter rate - 25gr