Fenwick rods with an 'FL' model number means the rod was custom/amateur built from a rod blank. Fenwick factory rod model numbers were based on the rod length in feet. Fenwick rod blank model numbers were based on the length in inches. For instance, an FF856 would be an 8'6" factory rod for a 6 weight line. The corresponding model number for a blank would be FL102-6.
Early Fenwick rod blanks were yellow and one piece. These blanks could be purchased as is, or fitted with a Sizematic metal ferrule. In the mid-to-late 70s Fenwick began selling the same brown, tip over butt ferruled blanks used for the factory rods.
The Fenwick rod blanks were US made. If you buy a custom Fenwick in the US it was probably built out here. Because anyone could buy a blank, the build quality will vary. Often the market price of an un-built rod blank is the same, or higher, than a nicely done custom rod built on that same blank. An ugly custom rod can be a good opportunity to get the rod blank for a nice price.