The very first batch of 1E1s are a separate type from all the others. For this first type there is a rectangle of missing dots in the forehead. Although I noticed this difference sometime ago, I forgot about it. Rein has reminded me of it and studied it in detail.
This clay paper has the thickest coating, and in the type of back scans I use, the watermark is very difficult, if not impossible to see. I find use in 1943 and 1944, but mostly in 1945.
This paper has a thick coating. The water is not as visible as that of the 5cCL1Aa. I find these stamps used in 1941 and 1942, with fewer used thereafter.
This paper is a slightly coated 1E4. Pettigiani refers to this paper as 'Mate Lustroso.' The first block shows how the ink can smudge on the coated surface.
Here are comparison images for a range of used blocks of four and six. The dates range from 1939 to 1945. The four papers seem to have the 1E4 watermark. The major difference between these papers is the coating applied to the printed surface.
This image shows the four papers at a higher resolution.