Update: With the help of a fellow IIS member (linkpng) who owns one of these AOG's and focal reducers, thanks mate! I've now managed to attach the focal reducer lens to the OAG. The lens cell part of the focal reducer unscrews and comes out and screws on to the end of the OAG (camera end) with the spacer ring removed. All you would need to do is work out the focus distance from the focal reducer lens to camera sensor. So now they can be used together for guiding
The photos below show the focal reducer lens on the OAG. The first two photos without the space ring, the last photo with the spacer ring on that then screws on to standard thread T-rings. You may still need a spacer ring or two to get the correct distance depending on what camera is used ie DSLR with flip mirror, mirrorless or one shot astronomy camera.