This is a bullnose prop cutter for dremel (or equivalent) multi-tools. There's holes for cutting props from 3 to 7 in in steps of 0.5in so: 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7 in.
Updated version (160309) is 5.5mm higher so more dremel tools can be used.