This is an express letter sent in 1935 by the railway post line number 7 Warszawa - Wilno (now Vilnius, Lithuania) to Königsberg (now Kaliningrad, Russia), Germany. I'm not sure where exactly this letter has been sent from but the sender's address is on the back and it's Slonim, today a town in Grodno region, Belarus. This TPO line 7 ran from Wilno (Vilnius) - Grodno - Białystok - then to Warszawa, so my guess is that it has been sent from Grodno then changed the line at Białystok then to Königsberg via Grajewo. Two arrival cancels on the back as well, one is Königsberg - Telegraphenamt and the other is Königsberg 5.