No - I handn't heard of it before either. Unfortunately, as far as I can see, Jenny Thompson doesn't give an illustration either, although by their very nature butter dishes always come with a lid (keeps the flies off, don't you know). I had to go to the shorter O.E.D. for a half decent explanatoin of the word 'trencher' - and wud seem that its one of those historic thingies - and which seems to derive from an origin whereby food - or kinds of food - were served in, or kept on, a circular or narrow platter - oftern made of wood. I suspect that in Elizabethan times they might not in fact have had a lid (they probably eat the flies) and it was kept on a wooden dish.
cheers Paul S.