Mountfield Emperor 21" Model No 83306/7 Drive Shaft Connection?

I have just renovated this mower and assumed (when I dismantled it) that the connection between the engine PTO shaft and the Drive Belt shaft was an interlinking set of 'prongs' on each shaft. However, it is obvious now that the connection is made by a bolt-type item (which I cannot locate!!) that presumably slots into the 2 half grooves - one in the end of each shaft.
Is this correct and, if so, would a suitably sized bolt suffice or is there a more specific part?
Nothing is shown on the Mountfield Parts diagram (Part no 61)