Setting up some cantis and didn’t keep track of what this spring was for. This makes the most sense to me but I don’t recall using a spring here in previous setups.

Gray piece does not make sense there actually, and I have no idea where that came from now either

Rear derailleur! I'm an idiot. Saga over. Case closed. Thanks for reading.

I knew someone will solve it (I guessed DT cable stop... swore I saw it on one of my bikes)

Its clearly not even a barrel adjuster so I'm not sure where I was going with that.

I have a 1965 Fender Jazz Bass- One day I noticed that one of the little saddle adjustment screws was missing- no idea when/where it happened- and it's a tiny little screw. The thought of putting a shiny new screw on that bass chafed me, and the thought of the expense of finding an old screw chafed me. I'd been thinking about replacing the bridge anyway- so I replaced the bridge. I had quit the band I was in- and then a year or so later they wanted me back, so I started playing with them again. One day we're sitting on the floor in the basement rehearsal studio, and I'm mindlessly playing with the carpet strands and I find this little metal piece- and I almost just flung it across the room- just because... I noticed it was threaded and had a single slot and HOLY **** IT'S THE ADJUSTMENT SCREW FOR MY JAZZ BASS!!!

No, it’s a barrel adjuster for either in the frame, the rear derailer, or a shifter. The spring keeps tension on the barrel so that it does vibrate. The picture below shows the frame mounted version.

93980011 by Stuart Black, on Flickr

Here’s the adjuster close up for an XT mountain shifter. The spring (red arrow) goes over the threaded barrel (green arrow) and then goes into the plastic knurled nut (yellow arrow).