The devil's in the details... FI-15 is 35/34mm cross-section while the E is 31/33.3mm. Gives you an additional 2mm on top to snick the cover home! Good to know the newer version F'tech's work as they now feature a one-piece nose whereas previously the entire body & nose was split and could be separated like many lesser-quality DIY FIEC's today. As I mentioned above that one-piece nose makes ALL the difference!!!
Statements in my posts are opinion only, not to be construed as fact. Any projects I engage in are at my own risk! Their outcome cannot be assured and may result in success, small/no change or catastrophic failure. I encourage no one rely on anything I say or do as gospel and to realize your mileage may vary!