Denso Compressor Question

[Ezzz]

Hey guys, riding in my truck feels like so I'm trying to get the AC figured out. My 69 C10 has a 2006 4.8 from a silverado that came with the compressor. Initially it didn't wanna fit so I took a bolt off the back for a little extra room test fitting and misplaced it. Now everything fits, but I can't find any diagrams saying what the thing is called. Obviously it's a bolt, but it also looked like a breather. There was a tiny hole in the bottom where it goes in the compressor, and at the top it had two holes that looked like the let air out but not in? I've attached a pic, but if anyone knows what it's called, or better yet where I can find one I'd really appreciate it. Thanks