As a potential buyer I'd be very interested in learning just exactly why this was done. Hopefully some Tartan owners will have heard of this done elsewhere and know the 'why'.
This may have been done to address some serious issues with the C/B system and if the seal was not perfect, or the C/B has been stewing in some trapped soup all this time there's no saying what you'll find.
This may as simple as using a router or a zip cutter to reopen the slot - but then I'd worry about the seal between the 'skin' and the keel/Cb box itself.
Surely there are other T37s available that have not been altered this way.
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)