OPF-FN509T, RMR is a solid state (one piece), steel plate designed for the FN 509 Tactical and FN 509 MRD. OPF-FNT09T, RMR is a one piece, billet machined plate for RMR/SRO, it is not guaranteed to work with non-Trijicon sights that have the RMR footprint. The way FN has designed the optics plate for the 509 Tactical is rather unusual to us, it consists of a MIM or cast steel plate with lugs for the RMR, and a larger plastic piece sans any lugs or means to prevent the sight from moving laterally. Factory FN 509 Tactical sights remain compatible with OPF-FN509T, RMR and provide lower 1/3 co-witnessing. OPF-FN509T, RMR includes the adapter plate, 2 6-40 x 1/2 T10 screws (same Mfg: Forward Controls Design