You have a 20 double-hole Hohner Echo tremolo harmonica, made circa 1930-1950(?). That's a guess.
It has the "art deco" style covers, popular from about 1933-1940, designed by art deco artist-designer John Vassos for the Chicago World's Fair Trade Exhibition(title?) in 1933-34. This may be one of Hohner's designs after 1934, not a Vassos design, but it's still in the art deco style.
Tremolo harmonicas are designed with two same-pitch reeds vertically stacked, each reed in it's own chamber. They are diatonic (one major or minor scale) harmonicas. The first six modes plus the 12th mode of the one factory-installed scale are also available, without bending or overblowing techniques.
One of the two vertically-paired reeds is slightly de-tuned at the factory. When the two vertically-paired reeds are played together, a wavy tone (tremolo) is produced, without performer-controlled techniques.1. Where should you start?
With an instruction book, and a website for tremolo harmonica players. The book:
"Tremolo & Octave Harmonica Method," by Phil Duncan, Mel Bay music publisher. Order it at your local sheet music store or musical instrument retailer, or national chain bookseller.The free website: http://www.thetremolo.ponderworthy.com2. Will your music sight-reading help to make learning the harmonica easier?
If you use traditional music notation on the guitar, some of that knowledge will transfer to the harmonica, if you use traditional music notation on the harmonica.
If you use only guitar tablature, no transfer will happen. The tremolo harmonica is a different musical instrument than the guitar, and the tablature systems are different.3. What year was this Echo tremolo made?
This was described in the first paragraph above. If all of the parts and case look like they are original (in your pictures, they look original), then the vintage would be as above.
One more topic: the reed placements of the 20 double-hole tremolos are in one of two placements. These are discussed more completely in the book listed above.
On a key of C, 20-double hole tremolo, some tremolos start hole one exhale (left side lowest pitch on the harmonica mouthpiece) with an E. That's called the "mi" reed placement, for the 3rd note of the major scale.
The second reed placement system starts at hole 1 exhale as a C note. This is called the "do" reed placement. Both reed placement systems are in the key of C, unless otherwise stamped or painted on the harmonica.
Happy New Year