KC, the glazing is too thick so it is indented but smooth. I just remembered I had posted a picture of the mark on an English website ….. “Let’s Talk Pottery”, ….. and lucky for me I searched said and found those pictures, so here is what I posted about them, to wit:
I took 2 pictures, out on my covered porch, in natural light.
By angling the camera, the reflected light caused a shadow to appear of what looks like the base of the “mark” ….. which the mark itself appears to be “a tapered/flared vase with a round base”.
See 1st photo, mark is in center, with base of mark extending into the light area and top of mark (darker shadow) pointing to upper left corner. The light area of the photo is the “blue sky” reflection from underneath the porch roof behind me.
In 2nd photo, mark is in center, but one can barely make it out.
Hope you can make out what I attempted to define.
Oh, and an added note of possible interest, being the designer/inventor that I am, I figured out from my above success that what one needs for such tasks, instead of or in addition to the “pencil/crayon trick, powder trick, etc.”, …. is a small battery powered flashlight/spotlight thingy which creates/generates a narrow beam of polarized light (like natural light). With polarized light, all the light rays or photons are moving parallel to each other so they produce a very distinct or precise shadow, just like Sunlight does. And you can see by the results in the following pictures which can not be seen in the above picture of the bottom of the platter.
Hope that helps for identification.
Sam C