precision 3660 m2 mount screw

I had purchased a Dell precision 3660 a few months ago, but when I tried to add a couple of new M2 SSD drives, I discovered the m2 mount screw for #2 and #3 slot are different and missing parts.  As a result, I am unable to secure the M2 drives.  How could this be missing and/or a mismatch.  How could I get some spare screws or stand offs?