Hello,
Photography had an impressive an explosive growth in the first part of the 20th century. Kodak opened it to the masses in the late 1800 - early 1900s and by the 1930's it was a common activity. Kodak was the biggest player with an impressive range of offerings from the professional to the casual amateur and of course the world's biggest supplier of film.
Thanks to advancements in shutter and lens designs, by the mid '30s there were offerings with multiple shutter speeds, faster and multi-element lenses and the proliferation of multiple film formats, etc. Bellows were a common design choice between lens and film plane as it helped tremendously in the portability of cameras.
In 1935 Kodak launched a new series of cameras called the Kodak Junior six-16 and six-20 using 616 and 620 film formats respectively. A few years earlier, Kodak launched these formats which are variations of the original 116 and 120. Let me know if you are interested as this would be a complete separate topic. Keep in mind that contact printing was still the norm, rather than enlarging, so the prints were the size of the negatives. Hence the multiple choices of negative sizes.
Anyways, this Kodak Junior series was made until 1940 but it gets interesting.
This is my own research on the topic.
The original Juniors six-16 and six-20 offered 2 lens choices: the top of the line f6.3 Anastigmat, and the lower spec Doublet lens.
The series II was launched in 1937 with 3 lens choices: The top of the line f6.3 Anastigmat, the mid tier Bimat lens (f11-f32) with both of these featuring a Kodon shutter with shutter speeds of 1/25, 1/50, 1/100, B and T. The lower spec model offered a "single" lens and only 2 shutter options: Time and Instantaneous from a Kodo shutter.
July 1937 Kodak Catalog
In 1938, the Series III was launched as an upper tier to the concurrent Series II. The Series II was only available with the Bimat or single lens. The f6.3 Anastigmat became the mid offering in the Series III between the f4.5 and f8.8 offerings.
However, in the October 1939 catalog, only the Series II is available with the lower 2 lenses. The Series III and the f6.3 lens were not offered. They were still showing in the March 1939 catalog.
My specimen:
I got this many months ago but hadn't have a chance to work on it.

Looks complete. Hard to see in the pics but it is extremely dusty inside and out. Also very stiff.
This is the less common Series II with the f6.3 lens so that puts it around 1937 or early 1938.
I have been able to find only 3 other identical units online with legible serial numbers. Another mystery as two are five digits serials: 51xxx and 55xxx while another is six digits 105xxx like mine which is 106xxx
So I can't yet establish any relationships with serial numbers, other than mine is higher.
Time permitting I'll restore this one up. Of course with the intention of using it.
Stay tuned...
Thanks,
Ismael
P.S. Still reading this? Boy this post is looooong!!!