The "Kenshin Kaden" basically refers to the last extra chapter (comes after the end of the manga and after Yahiko No Sakabatou) of Rurouni Kenshin entitled "Cherry Blossoms in the Spring Time" (Haru Ni Sakura). 
It was published in the Kenshin Kaden Artbook. 
(Source: The-OrO.com)

One hundred and forty years ago in Kyoto, with the coming of the American "Black Ships," there arose a warrior who, felling men with his bloodstained blade, gained the name Hitokiri, manslayer! His killer blade helped close the turbulent Bakumatsu era and slashed open the progressive age known as Meiji. Then he vanished, and with the flow of years, became legend.
 
(Source: Viz Media)
 
Included one-shots: 
Volume 1: Rurōni -Meiji Kenkaku Romantan- (second one-shot for the series, which Watsuki calls a "side-story") 
Volume 3: Rurōni -Meiji Kenkaku Romantan- (first one-shot for the series, which Watsuki calls the "pilot") 
Volume 6: Sengoku no Mikazuki (Nobuhiro Watsuki's debut one-shot) 
Volume 28: Meteor Strike

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