a) It wasn't in March 1939, it was October 1938.
b) The map does not show that land grab.
a) 1939 III is only the title of the caption and presumably represents the state of affairs, not the actual events,
in (March?) 1939. The caption itself reads
"1938 Aneksja Austrii i Sudetów przez Niemcy. Przyłączenie Zaolzia i Jaworzyny do Polski"
b) If you flick between that and the previous map, there is a definite enlargement of Polish territory
where I understand Zaolzie to be. If it's not that, what is it?