Aerodacious is a helpful website, but for details you should invest in the American Air Mail Catalogs from which the Aerodacious website draws it information.
Photos would help on your flight covers but from your description it sounds like an ambitious collector was trying to create something that was not official. The official first flights out of Fairbanks happened on May 3, 1938 and were flown by Pan Am subsidiary Pacific Alaska Airways under US Post Office contract FAM 15/16. This was the airmail route between Fairbanks and Juneau via Whitehorse. First flight covers were prepared for this inaugual flights. They flew Lockheed model 10C twin engine aircraft - better known as Lockheed Electras - on this route. (fun fact - Amelia Earhart was flying an Electra aircraft on her around the world flight when she went missing). On June 20, 1940, Pacific Alaska Airways was won a second US Post Office contract to extend FAM 15/16 airmail service to fly the US mail from Juneau south to Seattle via Ketchikan. First flight covers were prepared for these flights. Since Fairbanks had already had its inaugural flights in 1938, it is considered not to be a first flight resulting from the 1940 extension. Rather, your Fairbanks to Juneau cover would be considered as ordinary airmail on a previously established route, and you cover to Ketchikan would we considered as an odd variation on the FAM 20 first flight from Juneau to Ketchikan.
In addition to the FAM listings in the American Air Mail Catalogs there is also an Alaska section that lists many other Alaska specific first flights. And if you are interested in Canadian first flights the latest volume of the AAMC has just been released and focuses entirely on first flights and air mail stamps of Canada and Newfoundland. This just released 635-page reference book with countless color photos is Volume 4 of the 7th Edition of the AAMC and can be purchased at the American Philatelic Society website.
If anyone is interested in first flight covers I urge them to consider joining the American Air Mail Society at https://www.americanairmailsociety.org/