Its skids for your quad to protect the legs of the quadcopter when you land or crash. Designed to fit normal mounting patterns for 16-19 mm. Adds about 1,7mm on the screw length which when used with softmounted motors will add up to 2mm total. Add that together with frame thickness and you will know your screw length.
There are two versions, one with a hole in the middle for motors with a screw holding the bell (so you can unscrew the bell for easy service), and one without a hole for motors that dont.