Rob,
There were a few Mk3's and maybe some Mk2s [nobody knows how many] that were produced as shoal draft Top Hats. As far as I know there were no Mk 1's produced with a shoal draft.
What Formit did was put a foam plug in the bottom of the mould [it was about 300 deep] to reduce the draft.
As a result extra[?] lead was put in to compensate for the the reduced draft and righting action.
These shoal draft Top Hats have no water tank below the cabin floor [its all ballast].
If you want to see one, look out for Shaun and BLUE MOON, as his junk rigged Mk3 Top Hat is a shoal draft version. [Talk a bout wanting to be different!!!!]
Phillip.