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.

About This Item A drawer full of tools in the palm of your hand! • 36-bit set specifically for guitar hardware • Compact no-spill case, only 5' long • Only at StewMac!
If there's a set of tools every guitar tinkerer will love and use every day in the shop and on tour, this one has gotta be it! A great gift for shopmates, bandmates and guitar collectors.

SAVE when you buy the Guitar Tech Screwdriver and together! This set is a must-have! Be ready for practically any hardware on any guitar, from tuners to bridges, tremolos to pickguards, pickups to tailpieces, and more. The Guitar Tech Screwdriver Set replaces a whole drawer full of tools. We've included 36 essential bits for fretted instrument hardware, plus an easy-grip handle with extender. The entire set comes in a durable compact spill-proof latching hard case that fits nicely into your toolbox or guitar case (take it along to your next playing gig).
Bha Bscrew 4.5 For Macbook Pro
You won't find it at the hardware megamart, or anywhere else but here at StewMac. Contents: 6 Phillips bits: 000 • 00 • 0 • 1 • 2 • extra 1 11 Allen bits:.050' • 1/16' • 5/64' • 1/8' • 5/32' • 1.5mm • 2.0mm • 2.5mm • 3.0mm • 3.5mm • 4.0mm 11 flat-blade bits: 0.8mm • 1.2mm • 1.5mm • 1.8mm • 2.0mm • 2.5mm • 3.0mm • 3.5mm • 4.0mm • 5.0mm • extra 3.0mm Capture bit: 5.0mm non-slip flat-blade bit helps protect small screws and finished surfaces. A StewMac exclusive! 'The capture bit in this set is freakin' awesome! How many times have I had to fix a ding because my old driver slipped.'