Recent 65th anniversary of the issuance of the U.S. IGY stamp

The U.S. IGY stamp was issued on May 31, 1958. Recently we saw the 65th anniversary of that date. Happy, birthday, 1107! Congratulations on becoming a senior citizen.

I recently bought a cover which contained 5 of these stamps. 

Cover IGY 254 (front) from my collection

Cover IGY 254 (back) from my collection

This cover has several notable characteristics:

1. It is franked with a block of  four IGY stamps (Scott 1107).
2. The block has a first day of issue (5-31-58) cancellation for this stamp.
3. A single additional IGY stamp on the cover was cancelled (using a barrel/football) with a postmark from Quaker Hill, CT, on 7-26-1958. The cover was addressed to Mr. and Mrs. L.C. Maples in Quaker Hill.
4. Typed on the cover: "Carried on USS Skate (SSN578) on first under-ice crossing under North Pole. Achieved 12 August 1958." 
5. The cover is signed by James F. Calvert, the distinguished commander of the Skate.
6. Typed on the cover: "Courtesy of F.G. Weigle", an acoustic physicist who sailed on the Skate. I'm not sure exactly what "courtesy of " means in this case; perhaps he typed on the text and secured the signature.
7. The front includes the Artmaster IGY cachet (Mellone 4).
8. On the back of the cover is a nice cachet noting the USS Skate's polar ice cap cruise during the IGY.

The USS Skate (SSN-578) was: the lead ship of the Skate class of nuclear submarines; the third nuclear submarine commissioned by the U.S. the second submarine to reach the North Pole (8-11-58 during this voyage, after the USS Nautilus had done so in 1957); the first to surface at the North Pole through the ice cap (in 1959); and the first to make a completely submerged trans-Atlantic crossing.

The history of this cover is not 100% clear to me. I assume the International Geophysical Year FDC was produced first. The cover was postmarked again on 7-26-1958 in CT, near the ship's home in Groton. This date was before the Skate departed for the North Pole  on July 30. But the text says the cover was carried on the USS Skate's voyage underneath the polar ice cap. So I don't understand how the dates mesh. Could it have been postmarked before the ship departed, traveled on the ship, and then delivered after the expedition without an additional franking? Or sent on the ship without a postal marking, managed by Weigle, and then mailed to Quaker Hill upon its return (I lean towards this interpretation)? Or could there be another explanation? It may be worth noting that the submarine base in New London, CT, the Skate's base, is only 4 miles from Quaker Hill. Thanks for any suggestions.

I appealed for help to better understand this cover to the Facebook group of the Universal Ship Cancellation Society, and although I got some appreciation of the item, no one completely answered my questions. If any of you, dear readers, can shed additional light, please let me  know.