The original on Kristy was an AquaSignal Kiel (Keil?) Light. Mine wouldn't hold the bulb in place any more. The newer models were redesigned and parts are not interchangeable, AquaSignal doesn't have any of the old ones. The part that remains on your mast has an inside diameter that is a perfect fit for a PVC pipe fitting, sorry, I don't have the exact size. I cut a 1" long piece, and bolted it through the existing fixture to hold the bulb in place. Looks like it belongs there. I think I posted a pic of it a few years ago in the "Kristy" folder. If you can't find it, let me know and I'll post it.