There are three major printing types for this time period. The first printing, for which the earliest specimen I have is dated in June 1937, is light brown and printed on the 1E1 paper.
The second type is a red medium brown, and dates from early 1938. It is printed on the 1E1 paper and shows various stages of deep plate wear, especially on the face of Moreno.
The third type looks a lot like the second, except it is printed on the CL1A paper, the first clay paper. This paper is not as thin and shiny as the CL1B paper. An early specimen, from August 1938, is shown below.
The backscans of the CL1A are opaque.
For additional specimens please check this link