This is a connector for the deflector dish in 1/350 scale. It's to help in converting your Round2 TOS Enterprise into a Scout/Destroyer/Hermes/Ptolemy class. This one is based on fan designs. It aurments the lower dome on the kit instead of replacing it. It fits perfectly with the kit's lower dome and dish.
I also make one based directly on Franz Joseph's designs in the Star Fleet Technical Manual
available here.