DSC, Obviously I haven't made myself clear, or we perhaps we are talking about two different things.
Once I had identified the first word from the unclued letters I knew immediately what the theme was, and I was pretty sure what the letters, and therefore the grid, represented. As I'm a bit clueless working with the this type of theme, I went straight to Wikipedia. I expected the position of the letters/symbols to conform to something appearing in the Wikipedia, but they don't, hence my confusion.
The grid represents something other than what appears early in the Wikipedia entry. A hint from a friend reminded me of an alternative interpretation that I had overlooked. Once I ooked at the grid in this new light I could see that the letters are correctly placed. The thematic link from extra words in the down clues is a bit general. I'd have preferred something a bit more precise maybe two thematic links.