Ours are under the thwart. Bolted in place. Winder tales mk1. Happy hunting....

In Winder hulls up to around 3627 they were under the drop thwart moulding when built.
 
After around 3627 they are more visible under the case capping in the crew area. 

I think on 3612 they are bolted under the centreboard capping at the front.  

As a previous owner of 3612 I understand when she was new Simon Blake tried different locations for the extreme amount of lead and found (somewhat surprisingly as you might expect the hull balance point would be best location) that she went fastest with it toward the front of the cb case.  Since then most Winder boats also have it further forward than previously.  Has anyone tried more recent experimentation or can explain a logic re best positioning for the lead? 

Geoff, my theory is that the centre of buoyancy of the immersed hull of a modern Merlin Rocket is further forward than it should be ideally. That is due mainly to the rise of floor measurement point and the desire for minimal rocker in the aft run. So if the C of G of boat plus crew is aft of the centre of buoyancy, then the boat squats by the stern. Putting the correctors in a forward position helps to reduce this imbalance in a small way.