Trivet? ID help please

[Bill Scott]

I was at my favorite antique stop the other day and was talking cast iron with the lady who was working the counter.
She showed me this piece and wanted to know if I knew anything about it. I told her I didn't know much outside of skillets and griddles but I would take pics and see if someone could ID it for me.

The only marking I could find is the "D" on the underside in the third pic.