From the picture it looks like an EliteDesk mini rather than a SFF (but correct me if I'm wrong). If so, that's a standard 100mm VESA mount which uses M4 screws.
I mounted one of these under my desk a few years ago, and used a VESA quick-release adapter (one half screwed to the desk, the other half screwed into the EliteDesk mini. Makes tinkering much easier.