"Authentic", published by Hamilton & Co., started as a series of paperback books carrying a single novel-length story and evolved into one of the more intriguing British magazines of the 1950s. It ran for 85 issues from 1951 to 1957, changing its name to "Science Fiction Fortnightly" with #3, "Science Fiction Monthly" with #9 and back to "Authentic Science Fiction" again from #13. In quality, whether of fiction or of artwork, it rose far above the juvenile-oriented Spencer magazines of the same era and became a far more complete and rounded magazine. Even the very early single-story issues began to carry readers' letters, science articles and editorial comment, though it did not complete its evolution into a "proper" magazine until #26, when a six-part serial by Sydney J Bounds commenced alongside the main novel.
There was a very strong emphasis on science fact under the editorship of H J Campbell - a strange parallel with Astounding under John W Campbell, but there is no relation as far as I know. By the middle of the magazine's run, it was often carrying more features than stories and many of these were illustrated. For a while, the magazine included a bound-in feature supplement on glossy paper allowing photographs and higher-quality illustrations.
For six issues (#29-34) the magazine carried photographs on the back cover, stills from SF films of the period. It then carried a series of covers called "From the Earth to the Stars" illustrating man's imagined progress in the exploration of space, with an explanatory text on the back cover. These ran from #35 at least until #44 ( I haven't seen all of the back covers, so I can't be more exact). Another distinctive feature of Authentic was that many of the front covers wrapped over onto the spine. A long series of front covers were done by the art editor John Richards, a few under his own name but many under the pseudonym Davis. According to Harbottle and Holland*, he also did some interior illustration under the pseudonyms Gerald, Mallory and Muller as well as Davis.
The editor in the magazine's later period was the well-known British writer E C Tubb and the emphasis shifted markedly away from the non-fiction features towards fiction, though it retained an editorial and book reviews until the end of its life. Finally, interior illustrations were eliminated to increase the fiction wordage, just before the magazine ceased in 1957.
*British Science Fiction Paperbacks 1949-1956: an Annotated Bibliography and Guide; Philip Harbottle and Stephen Holland, Borgo Press 1995.