Riding on the freeway last night in heavy traffic, I got my rear wheel clipped by a cager changing lanes behind me. My bike briefly did the two-step before launching me high-side style into low-earth orbit. Thanks to the talented engineers at Sidi, Shark, Knox and Furygan, I walked away with minor rash and what CT scans proved only to be a gnarly hip contusion rather than a bonafide fracture this time - thank God. Sadly, my K6 1k didn't fare as well. The mainframe cracked on both sides in perfectly straight, vertical lines, right along the frame weld midway down the horizontal section of each (L & R) spar, directly beneath the tank. Welding it crossed my mind, but its a gamble, no guarantee it'll be safe, and honestly kind of expensive considering how much a straight used frame fetches. Looking at Ebay options today i keep running into frames that have an "F" designation. All my years (30) with these bikes (GSX-Rs) and I've never come across this moniker until now. Can somebody enlighten me as to what for example makes an F12 frame different from an F13 or F15?? I can't find any info on this in the places I've looked. Does it have something to do with the condition? Salvage status? Regional design variation? I'd appreciate any input here. Thanks in advance.