These are fascinating stamps.
There's a very useful booklet on these stamps and the various maps that were used that was published by John Barefoot
back in 1983.
At least 65 different maps were used. Full (or almost complete) maps with stamps are known using 61 different maps and blocks are known from another four maps.
Each map was used to print 228 stamps in 12 horizontal rows of 19.
Most of the maps used were inscribed in Polish with others in Latvian (even though most locations were in Latvia or Lithuania) and there are marginal inscriptions in German.