Standoff need for Precision 3660

Hi, 

Apologies if this has been answered but I couldn't find the answer. I have also reached out to dell but I am not getting an answer.

We have just purchased a new computer for the office and wish to add more storage. We seem to be missing a standoff, but I am getting mixed messages about which I need to order. I've checked the manual for this model and it seems the screws are m2x3.5 but I cannot seem to locate which stand-off I need. 

I went to a local store and they opened up a similar but not exact model and said I need this at the link below. However, this differs to the standoff that is already installed in the second picture (SSD2). 

https://www.dell.com/community/Inspiron-Desktops/Inspiron-3670-M-2-mount-screw/m-p/6114118#M6047

Can anyone help determine which standoff I need to order for the first picture (SSD1), please? And whether they will also use the standard screws to install the actual SSD. Thanks in advance.