Now, isn't it the dream of many an instrument maker: to make a saxophone with only open keys? I mention Sax, Leblanc and Loomis... they all did and so did I. A couple of years ago I started more or less by chance with an experimental alto, to be followed by an already less experimental tenor. Recently I finished a soprano, the most complicated of the three thus far, the most sophisticated.
Open key mechanisms have the advantage that tones are more even in sonority than in the conventional saxophone, which has a number of closed keys which to a certain degree muffle sound and which create uneven intonation characteristics. The D, for instance, is a well known victim of these tendencies. This is because the neighbouring Eb and (long) C# are open configurations whereas the D is kind of a fork.
Open key mechanisms also offer the exciting possibility to add fingerings to the existing system. For instance, a second "P" (petit) can be added for the 2nd finger right hand to make an Eb in the same way as the "P" for the first finger left hand can make a Bb. Also, a Db-spatula can be added in the right hand fourth finger plateau, operating a one-finger Db or C# which can be lowered to B or Bb by the fourth finger left hand in the normal way, or which can be used for a D - C# movement using one finger only. The Db or C# can then be taken by both right and left hand, either as flattening the D or as sharpening the C.
Similar fingerings have been tried in the past (by Buffet-Crampon and Leblanc among others) but with little success. In my opinion this was also due to the cumbersome mechanisms used. Also, open-keys saxophones in the past, like Loomis' or Leblanc's, often used parallel tone holes to obtain their objectives, with ever more pads and disturbances as a result. My open key saxophones use switches on one and the same key and so avoid the use of these parallel tone holes. These switches feel slick and operate swiftly and have in the mean time proven to be quite reliable.