Today (Sept 10, ’16) I drilled the holes and added the new rails. A couple of the holes were slightly out of alignment but the rails now fit and look good. I have to countersink the coachbolts, and one hole in the coachroof has lost is thread. i will fill the hole with resin and cut a new thread. Then I can cut the rails to length, round off the ends (is that “chamfer?” I shall check!) and perhaps round off the edges as well.
I do not think there is enough depth to the rail for me to make and put caps over the bolt heads, but if there is, I will use the spare 12″ of teak to make them. Small circles of teak to sit over the bolt heads and hide them.
The next decision is – oil or varnish? Dunno. Oil is more practical, varnish more beautiful.