"cents" Cover to Belgium Undated, postmarked in May 1939?

This cover was mailed to Belgium. From the stamps used, it is likely a 1939 cover. It is difficult to determine if this is a philatelic cover, since it does satisfy a specific rate: 20 centavos ship rate. Here are quick lines to pages for the three major groups: philatelic exhibition covers, covers that are quite likely philatelic, and covers that seem to represent regular use.

The cover bears singles of the 05c1E, 2c1E, as well as two 1c1E stamps.
Below is a detail of the 2 1/2 centavo stamp.
The cover also bears singles of the 3cSMGY1E, 4cGR1E, and 6c1E, shown below.
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