The Twenty-one-cent Prexie
Multiple rate use
Part of a wrapper sent in 1945 for seven times the three-cent first class rate.
As there is no mention of airmail, this cover was most likely sent double weight at three cents per ounce plus fifteen cents registry to Canada in 1942.
This cover was sent in 1939. Although it was mailed in a penalty envelope it likely contained stamps for a collector, so was not Post Office Department official business, which would explain the need for postage and a registry fee. This would break down to double the three cents per ounce postage effective from July 6, 1932 until August 1, 1958, and the minimum registry fee of fifteen cents effective April 15, 1925 to March 26, 1944.
A thank you to Willy Knopf for pointing out the correct rating on this cover, and some other errors on this website.