Giant Floral Platter

This is a giant platter made in France in the 19th century or maybe earlier. I have owned it for many years and it just has me stumped! It is huge being 21.5 inches wide, 15.5 inches deep and 2.5 inches tall. Handpainted with a superb floral arrangement as well as a few smaller flowers and leaves. One leaf with a flower bud was painted over a large rim chip which I have never seen before on that scale. Two green bugs are also on the rim. The piece is in good condition considering the side - the one overpainted rim chip I already mentioned and another rim chip plus flaking to the paint. I first thought it might be Tournai because the porcelain has a green translucency but the style of flower does not fit nor do I think this is soft paste. The only markings are an impressed 18 and 20 underneath. Maybe someone out there has more of a clue what this might be than me :) It is a very ehavy piece! Please email me for more information or other pictures.