I picked up a horizontal mill arbor for fly cutter bits at an estate sale for cheap. It was the first time I've seen one of these. It looked a little larger than MT3, and it turned out to be just a bit larger, but not al the way to MT4. There is no half size, so this must be a BS9 taper. Is there any demand for these things? I am about to just turn it down to MT3 to fit my mill, but maybe that is a waste? It will be years before I spot another horizontal mill arbor.