Here is a super rarity; early FL (no USA) Serial Numbered Mint original alto piece.  The serialed FL are together with the double rings considered the best of the FL metal Links. From my perspective they are nearly the same as the FL double ring, but with a touch more baffle brightness and airspeed.  This is variable, of course.  Sometimes they are brighter, sometimes they are indistinguishable from double rings in terms of how they play.

An alto model, in mint condition is a rarity as well.  This piece appears to have barely been played, if at all.  It has a beautiful rail/baffle/tip finish.  TIME CAPSULE!