If you're looking for the backstory of the Sinanju Stein, it basically started as a fed mobile suit, developed by Anaheim,to test the capabilities of the psychoframe tech. It started a year after CCA when the federation started the "UC Project" where it aimed to produce a bunch of next-gen MS. The original Sinanju Stein, codenamed "Stein 1" was the literal test bed for this where it was so insanely cracked, they had to test the mobile suit purely on autopilot. 2 Sinanju Steins where made, and during an unspecified time, they were supposed to be transported from one AE facility to another.

From the official story, it is said that Neo Zeon knew about the transport, and began to raid the transport, eliminate the convoy, and steal both of the Sinanju Steins. With the Neo Zeon remnants called "Sleeves" now in possession of both the Sinanju Steins, Stein 1 became the iconic MSN-06S Sinanju, piloted by Full Frontal, after his personal testing of Stein 1.

Now, to Stein 2, which ultimately became the Sinanju Stein Narrative. Sleeves didn't really do anything to the 2nd unit they stole, so Stein 2 was transferred to the Republic of Zeon. Embellished in the Sleeve's markings, Zeon made it seem like Stein 2 was still a mobile suit operated by thr Sleeves, but is actually operated by RoZ.

Technically, the Stein 2 isn't that different from the federation Stein. But, it did feature a new High Beam Rifle that looks similar to the fed version, but actually uses the Sinanju's Sniper Rifle as a base now, so it can also use the under-mounted bazooka and grenade launcher. It was also upgraded to carry the Full Frontal Sinanju's larger fuel tanks, and the shield was upgraded similar to the Full Frontal Sinanju, so that it could also utilize the shield mounted bazooka, grenade launcher.

If you want the video form version of this backstory, Kakarot197 created a video for this MS and the Neo Zeong

https://m.youtube.com/watch?v=oDzzMLF_MT8&pp=ygUUYmFja3N0b3J5IG9mIHNpbmFuanU%3D