This is going to be difficult to explain, but with my D comet replica, which is a great bike to ride BTW, with a D type AVO rear shock, who ever did it, and I think it was commissioned by Ivor Simon, they got round the problem a different way, it still has the additional lower frame support running from the headstock to the F106, but where it terminates at the seat bracket end it has a heavy u channel incorporated that is welded horizontally across the frame, not longitudinally to the frame, I think from memory that the F106 is cut to fit inside the U channel, and then secured with three or four bolts with nuts and washers that bolt the U channel to the F106, the F106 is a perfect slide fit up into the U channel, and the two parts are bolted together with through bolts, washers and nuts, all done up really tightly and making for a very solid connection, OK not for the purists, but it works and is barely noticeable. I would photograph and post it, but I am current laying in a hospital bed, so unable to do so. Make more D Comets, they are great!