I reassembled a Mac Pro A1186 and have extra parts left over. What do they go to?

I disassembled a Mac Pro A1186 for cleaning and upon reassembly found that I have several screws and what appear to be some kind of nuts. I don't remember what they go to.

This system did not have any hard disks. I did not remove the only optical drive. Everything else was removed for cleaning including the motherboard.

I did not encounter any other similar screws during reassembly. It is therefore presumed that they are totally unique for their purpose.

The nuts are also unique and I'd have thought obvious enough considering their size and number to be remembered. But I can't find a place for them.

What do these parts go to?

For your personal entertainment I must also mention that that, although I am an experienced PC user, this is my very first Mac. It is very embarrassing for me to have reassembled a computer and not only have parts left over but have to ask someone else where they go.

For a while during disassembly I kept thinking how nice and obviously thought out everything was. But then all of the different parts and pieces started piling up and I knew that I was in trouble. And, well, here we are!

Answered:

As Tetsujin answerer already, the nuts are part of the locking mechanism that s initiated by a level at the back of the case. I had removed these while trying to figure out how to extract the power supply.

The other two screws, which I found to be identical to the ones which secure the 5.25” drives, secure the cage around the memory boards and secure it to the bottom of the case.

Solution 1:

I'm posting this as a pure guess, just so I can add a picture...

The underside of the drive bay has a lot of screws like the ones in your picture, & also the pic shows one of many areas, under the PSU housing, that uses that type of 'spacer' as sliding locks - just in case they're not actually spacers.
Picture is from the Mac Pro 3,1 Service source - the yellow-circled screws are from the PSU & are dome-heads. The ones similar to your pic, button-heads, are the non-circled screws.

The red-circled area is what I meant for the 'spacers' - drive locks perhaps, activated by the release handle at the back of the Mac [I've 3 different Pros here, but all are in use so I can't take them apart to check ;)

I've never taken the mobo out of a Mac Pro, so never seen underneath to know what the spacers may look like.