Nemo, thanks again, I just realised I hadn't replied (I thought I had)
I can now carry on with my 'pin rails' knowing what they are for and what to incorporate into them. Any more photos of small details welcome.......
As for what I've been up to, well, a bit more planking.....
Before today's effort....
And then with masts.......
The masts simply slide onto threaded bars. There will be a nut on each bar so once the mast is on and the rigging is connected the nut can be turned to raise the masts and tension the rigging lines. The threaded bars go through the deck and then through 18mm wooden support plates (see photo above) these plates will be bonded between frames as well as to the underside of the deck. The reason for the strange shape (as apposed to a straight plank from one side to the other) is that I have allowed enough edge landing to be able to incorporate the rigging pin attachment points giving a total thickness of 21mm. That should be enough!
The threaded bars continue down to the keel where they will be fixed in position after alignment of the masts once the time arrives. U2