The C & F Chest Patch Threader holds your flies securely in foam slits. Lots of fly boxes do that. What others don't do, though, is provide sufficient ventilation for the flies to dry properly. Waterproof boxes in particular keep your flies wet! The C and F Chest Patch Threader has open ends, allowing flow-through ventilation. You can clip it to a D ring on your vest or use the pin on the back to attach it directly to your shirt or sling. The included Midge sized threader is a big help if you fish with small flies.
All of the C&F products I have seen are excellent in both concept and form.