This is a 3D printed ABS (acrylonitrile butadiene styrene - a tough, slightly flexible plastic) bracket that solidly holds a standard 9G servo in place. Attaching to the layout with two screws in slots, it allows for a solid mounting to wooden layout roadbed and yet still provides some fine adjustment for getting the throw arm position exactly right. This bracket holds only the servo, which is useful if you're using another manufacturer's controller or for mounting the additional servo from our "crossover kits" that allows for two servos on a single control board. See the ACC-MRS1MNT and ACC-MRS23MNT for printed brackets that hold both the servo and the MRServo control board.
As with all ISE products, our mounting brackets are open hardware. For those interested in printing their own, the design files and ready-to-print STLs are available on Thingiverse.