If you look carefully, on the capping piece nearer you, you can see 5 filled in holes and what appears to be an outline of some track?  Seen that before on other Merlins?  Was it something used to limit the position of the center baord in its "fully down" position. that someone has removed?  the board will go a good 20 degrees forward fo vertical at the moment ...

I'd say that's where the corrector weights were attached when the boat was underweight.

No, it's not that Chris.  Elusive, which is a Laurie Smart NSM2 in the same batch as this boat, had the correctors under the thwart (till they fell off).  I think it was a cleat for one or other of the spinnaker pole height or the control for the mast ram, and looks to have been moved once.  There was no control line for the centreboard, just friction pads, but originally did have a stopper on the handle of the board.

Yep, I can see where the stopper used to live on the board ...  the mast ram is brought back to the controls on the side of the thwart, and that looks pretty orignal.  

Like you said, the corrector weights are firmly fixed under the thwart, and they look pretty original too. I'm pretty sure it wasn't a simple cleat .. as in real-life as opposed to photos, I can clearly see the mark of a track around 12" long where the wood is less faded.

When you say "same batch .. " this one is Rowsell built,  or did Laurie Smart share premises or someting in those days?

Looking at the photos I thought it was a Smart boat from the centreboard case.  It's a contemporary Rowsell then, and much the same shape.