Mystery Bolt

[DCDO08]

I was changing the thermostat in my 2015 Sierra Denali 4wd w/ 5.3L. I found the bolt pictured. 

It has upper and lower threads which seems unique. Any idea where this came from? 
 

[TigerTruck]

That looks like the left bolt that goes through the AC compressor. The right bolt is a stud. Is it 13MM?

[DCDO08]

YES! Thank you so much. I was able to get it back in. I’m shocked it ever came out but even more that I was lucky enough to find it and where it goes. Again, I really appreciate your help. 

[TigerTruck]

Its hard to tell but it looks like the AC belt is rather new looking. Was it changed recently. This is a stretch belt which puts tension on the AC compressor. I doubt that it backed its way out. Did you have any other work done? Maybe lifter problems? AC compressor work? Those repairs require the AC compressor to be removed. Maybe it was never reinstalled. From the looks of the bolt being dirty and greasy, it looks like its been out of place for an extended period of time.

[DCDO08]

Dealer replaced a fuel pump last Fall. That’s the only thing it’s had done since I bought it in 2020 other than what I’ve DIYed. I’ve never touched a belt on it. Strange. 
You’re right it was filthy. Cleaned up pretty good.