This is the second and last batch of these stamps up for study.
I find the 1940 printing is on the 1E4 stamp (small horizontal RA)
In 1941 the second clay paper is used, with a more yellowish tone, on CL1B paper, thinner than the CL1A used in 1938
In 1945 the CL1B is reissued on darker colors.
The printings, as I find them, are:
1. 1937, light color, 1E1
2. 1938, dark color, 1E1
3. 1939, dark color, clay, CL1A
4. 1940, dark color, 1E4
5. 1941, light color, CL1B
6. 1945, dark/semi-dark color, CL1B