Coming back on track, I do wonder whether the ventilation slots (which are the key to the whole invention) are actually necessary. I treated myself to a rather smart rectangular vitreous-enamelled cast-iron casserole dish a few months ago, and have regularly used it since then to cook roast potatoes. Now as everyone knows, to get roast potatoes that are floury in the middle and crisp on the outside (i.e., perfect), you must avoid them coming into contact with steam; yet my roast potatoes turn out perfect every time, despite there being no ventilation slots in the lid, and I therefore assume that the lid/base seal is sufficiently less than perfect for the steam to escape naturally, without any need for explicit steam vents.
** Phil.