I noticed that the new adaptor doesn't quite fit in my version of the 27in iMac (late 2013 version). With the hard drive bracket in the position where it should be (indicated by arrow), the new adaptor bumps into the bracket (in area circled).

So I made a small modification to the new adaptor with a metal file.

Picture with modified adaptor (VESA mount) in place (the red circle around the modification).