Optiplex 755 USFF Part Number for HDD Caddy/RAILS ???

I have inherited eight (8) Optiplex 755 USFF (ULTRA SMALL FORM FACTOR) machines. They are all missing their hard drives and I just realized under the cover it's just a metal tray missing the blue plastic caddy/rails part. I'm having a terrible time trying to find the proper part number because everyone online tags their photos with about 10 different Optiplex 7xx models ranging from SFF to not specified and rarely can I find USFF. 

I know a photo is worth a 1000 words but if I had one I'd probably be able to find the part number. On the 755 Mini Tower it has blue trays or caddys that the hard drives clip into and then you slide that into the machine. On the USFF is it a set of blue rails that screw into the HDD or is it a similar caddy? I have piles of them laying around (from 755MT and Precision Workstations) but those blue trays don't fit the 755 USFF. 

To clarify a little more. There is a metal tray/box that the HDD would clip into. This metal tray bolts down to the chassis. There is a small square fan that screws into that metal tray so the part I'm looking for is the blue plastic part for the HDD only WITHOUT a fan, just the part that securely attaches the HDD to the machine. 

Thanks for any help. When I entered a few of the ST's into Dell's website they specify the HDD that the machines shipped with but do not specify the part number for the blue HDD caddy/tray. 

Thanks.