I find both unwatermarked papers, grid (NGR) and white opaque (NOP). The top pair in this image is a comparison NGR pair, and the five specimens below it are NOP, very rare.
With more time, I would like to study the Straight Rays printings. I wonder if this stamp was printed on the diffused paper of 1943. I find it used in the 50s, when the clear printing (2C) took place. This image is in true colors.
And this one shows the watermark more clearly thanks to the Photoshop 'equalize' filter. I only find one stamp on 1E4 paper.