The tremolo harmonica was invented by Wilhelm Anton Thie a harmonica maker in the 1830s and used the german Richter tuning. The tremolo harmonica is also very popular in Asia and for instance, in Japan we use a different tuning more suited to traditional folk music created by Shōgo Kawaguchi (川口章吾).
The tremolo harmonica is sometimes referred to as grandpa's harmonica since it was this old-style harmonica that was so popular in the 19th century. This harmonica is unfortunately very limited in the sense that you can only blow and draw notes and the note layout let you only play very simple diatonic melodies. Another specificity of the tremolo harmonica is its sound. Each note is produced by two reeds vibrating simultaneously. While one reed is tuned a bit low the other one is tuned a bit higher creating a modulation effect called tremolo. When you blow or draw a note the sound you hear is two reeds playing at the same time with a moving effect very similar to the sound of an accordion. The tremolo is still popular today because it doesn't require as much technique as the diatonic or chromatic harmonicas and can easily play simple melodies. The downside being that it is fairly limited.
The Echo 54/64 and the Echo 56/96 two classic models that are double sided tremolos allowing you to play a C scale on one side and a G scale on the other side
The Hohner Echo Harp 2309 is a single tremolo harmonica with only 9 holes
The Golden melody tremolo is the tremolo version of the Golden melody with golden like covers using a plastic comb to avoid swelling it has 40 reeds and 20 holes.
The SU-21H a 21 hole tremolo model available in many keys
The SU21M a high-quality finish of the SU-21H only available in C
And the very interesting and expensive SCT-128 the only Chromatic tremolo harmonica on the market. It is a fully chromatic harmonica with double reeds and the sound of a classic tremolo!