FSN OFA System Accessories. The FSN OFA Professional adapter for open systems is used for milling with a guide rail. Its intuitive adjustment and customization allow for easy handling during work. The adapter is suitable for numerous combinations of milling machines and guide rail systems. Additionally, its high-quality system components ensure reproducible milling results. This adapter is designed for use with guided milling using Bosch FSN guide rails. It is compatible with a variety of milling machines and guide rail systems.