Hi Peter, as Sweibertje says, it is the mast pivot pin hole and if you intend to leave the mast up his suggestion of a galvanised bolt (preferably a coach bolt with a smooth shank and thread only on the last half inch or so) will work fine.
If however you want to get the real McCoy, here is the P&B webpage from where they will sell you a stainless steel one for £18.30 plus carriage etc.
There is nothing special about it and if you could find someone to sell you a piece of 6mm dia stainless ( or I suppose aluminium?) then to drill a couple of holes in the end to take some rings should not be too difficult. In use the rig tension should be set up so that the weight of the mast is on its heel and the pin is not doing anything; in fact you should be able to rattle it in its hole. If you have not yet got one, buy a copy of the Wayfarer Book from the Association, a lot of questions are answered in there and it is ridiculously cheap for the wealth of information you will find within.
I do take my mast down quite a lot and for that the pin is invaluable, if you make yourself up a boom-crutch that sits above the transom so that you can rest the mast on it and so protect the spreaders, then lowering and raising the mast becomes a one-man job.