Man of Honour
If you place the back of the msa 20 on the back of the AW, how does it look? Does it look like it will fit or is it basically sitting there unable to actually slot in? (that is how it looks in my mind visualising the photos in a 3D space in my head). If the latter then yes, it is not the correct arm for the AW sadly. Personally I would buy just a normal VESA standards compliant arm (there are hundreds to choose from in every budget).
Edit*
I just googled the price of that arm and wow, over £100!! - That's Ergotron money. I would definitely get a refund on it and get a proper VESA 100x100 arm from Ergotron then.
Edit 2*
On the Dell page for that arm it says it has a VESA 100x100 interface, are you sure there's nothing extra in its box that allows you to fit the VESA 100x100 mounting bracket that comes with the AW onto it?
Notice bottom left, it says packaging contains a VESA mounting plate. If you fish that out you will be able to use the AW VESA bracket on that.
Edit*
I just googled the price of that arm and wow, over £100!! - That's Ergotron money. I would definitely get a refund on it and get a proper VESA 100x100 arm from Ergotron then.
Edit 2*
On the Dell page for that arm it says it has a VESA 100x100 interface, are you sure there's nothing extra in its box that allows you to fit the VESA 100x100 mounting bracket that comes with the AW onto it?
Notice bottom left, it says packaging contains a VESA mounting plate. If you fish that out you will be able to use the AW VESA bracket on that.