Special postmark on letter from El Salvador to UK?

Hello Specialists!
on the following letter from El Salvador to Liverpool in 1944 there are two blueish postmarks: the round one seems normal. but the rectangular one, is this something special?
Front:
Zoom:
Back:
many thanks!
Peter
on the following letter from El Salvador to Liverpool in 1944 there are two blueish postmarks: the round one seems normal. but the rectangular one, is this something special?
Front:
Zoom:
Back:
many thanks!
Peter