1948 MANILA CIRCULATING TAXED 20c BUT 40c PAID

MANILA CIRCULATING using 5c U.S. Postal envelope with US ARMY APO /
MAR 18 1948 postmark. MAR 20 1948 Manila Post Office entry,
taxed 20 CENTS as invalid use of 5c postal envelope. 
Total of 40c paid using 4c x 20 pcs of 4c Rizal Monument Pictorial issue,
tied with MAR 22 MANILA POST OFFICE / POSTAGE DUE PAID pmks.

 
The only reason I can think of on why an additional 20c was paid is the error in the "20c" Postage Due entry which should be 40c to include the penalty charge on the assumption that the cover is quintuple or five times rated.

The 20c Postage Due notice does not make sense since basic rate then was 4 centavos per weight increment. If exclusive of the penalty, technically the postage is 10c at 2.5 times weight which is not proper since 4c is charged per weight increment or fraction thereof and therefore the proper postage should be 12c and 24c inclusive of penalty.

6.11.2007