Flannel panel is a humorous term for a magazine

Magazine

Magazines, periodicals, glossies or serials are publications, generally published on a regular schedule, containing a variety of articles. They are generally financed by advertising, by a purchase price, by pre-paid magazine subscriptions, or all three...

 masthead panel that lists publisher and staff details.

The origin of the term is unknown, but according to the Oxford English Dictionary

Oxford English Dictionary

The Oxford English Dictionary , published by the Oxford University Press, is the self-styled premier dictionary of the English language. Two fully bound print editions of the OED have been published under its current name, in 1928 and 1989. The first edition was published in twelve volumes , and...

, one definition of "flannel" is: "Nonsense, ‘hot air’; flattery, unnecessary ostentation." Thus, the origin of the term likely comes from the perception that journalists are full of hot air and nonsense.

Usage

Short-lived UK computing magazine Amiga Power

Amiga Power

Amiga Power was a monthly magazine about Amiga computer games. It was published in the United Kingdom by Future Publishing, and ran for 65 issues, from May 1991 to September 1996....

points out: "It [flannel panel] is, of course, merely the list of credits at the beginning of a magazine, but, like all jargon, given a stupid elevating name so people who work in a particular industry can feel superior to people who don't."