The support pole on a Sidley rigid pole clips to fittings either near the foot of the mast (see photos on pages 6 & 27 of Guy Wilkins book 'Mirror Racing') or on the foredeck, just ahead of the mast. It looks like Guy had two fittings on the mast a few inches apart (no doubt to allow the height to be set). I'm not sure of the complete history, I understand having this fitting on the foredeck meant the height of the pole changed as pole angle changed and at the time this was thought to be an advantage. The fitting was often one of the basic plastic spinnaker pole eye fittings (Holt) Allen A132, see http://www.ukmirrorsailing.com/.../a_132_m_spinnaker_pole... and the support strut had a corresponding snap hook (Holt) Allen A31, see http://www.ukmirrorsailing.com/.../a_31_spinnaker_pole.... No doubt other combinations were used - it needs to give some freedom of movement, so the pole angle can change, but at the same time support the pole & (normally) allow the strut to be disconnected when the pole was stowed.
