This is the same Vixen Dovetail Plate Mounting Block that is originally supplied upon the Vixen Sphinx, GP2, and GPD2 mounts.  It is primarily offered as a replacement for a lost or damaged original dovetail plate mounting block.
However, if you own a Vixen Atlux mount and wish to use the Atlux mount with Vixen's standard #2661 dovetail tube plate or #2576 accessory plate, you will need to purchase this dovetail mounting block as well as Vixen's #8779 base plate for the Atlux mount.