'F' stands for 'Fiammata' (straight grain) and you can find three different levels, from F1 (the lower) to F3 (the best).

F3 is really hard to find and less than 1 piece every year is made.
Sometimes, we run across an F2, like the one presented in this and we get lost in looking at the beautiful and tight straight grain covering the entire pipe, with dense birdseye on the bottom of the bowl. Several of Vitaliano's dublins are finished with natural plateau rim.