I started off with the GG&G cantilever mount that I used on a flat top. It co-witnessed and put the dot right where I wanted it. The reason I changed was because I installed a KAC RAS II which has a "hump" where it attaches to the flat top. The meant that the cantilever was now too high and would not co-witness the sights. So, I called MSTN, told them what I had and they sent me the correct ARMs mount for use with the RAS II (which I also bought from them). It now also is at just the right height. To be honest, I don't know exactly what I have; I just took their word for it and it worked great. I do know that you have the mount itself, a spacer, and the rings.
If you emailed MSTN I bet you get an answer tonight. The fact that it is Sunday probably won't matter; they have GREAT customer service.