I think you may have what's known as a "Frankenstein" around this neck of the woods. The "FR" s/n prefix was used on a batch of frames (complete lowers) sold by Colt through CDNN a few years ago. At one time you could buy a couple of different complete lower configurations, and a complete lower in a Colt hard case with all the normal pistol manual and paperwork for less than 200 bucks. If memory serves me correctly, I think I paid 169.00 each buying them six at a time. I still have one of them waiting to be built into a 40 cal "Gold Cup". I have no idea how many of these frames were sold, but they pop up occasionally, with everything from WWII military slides to Gold Cup uppers on them. There was an assortment of new Colt series 70 slides being sold at the same time, and people were buying the frames and slides and using a mix of parts to build complete guns.