I'm pretty sure this is incorrect and the Polish and Latvian borders were lower during the Interwar Period. Maybe I'm wrong but I notice it the most when I see the city of Daugavpils.

https://www.researchgate.net/figure/Inter-war-Latvia-regions-Kurzeme-Latgale-Zemgale-Vidzeme-and-neighbours-Own_fig1_373430178

https://www.deviantart.com/klimantas/art/Map-of-the-Baltic-states-since-1918-945822501

https://www.maps4u.lt/en/maps.php?cat=36