The "FR" means "frame". There are a couple of different "Special Configuration" P226s thave have been released. Both are built on the MK25 frame (with corrosion protection, etc.) and the true mil-spec Picatinny rail, but have a different slide (no anchor) and usually different grips. One version has E2 ergo grips and another has rosewood grips.I noticed a site selling a " SIG W2269MM 9MM MK25 FR TB NS 1PC GRIP"
Any idea what it is?? Is it possible that Sig is offering MK with a threaded barrel?? Any speculation or actual knowledge would be greatly appreciated.