The frog

[oshfdk]

I'm still in the US and I don't have Photoshop on this laptop which is really annoying. I did my best with GIMP to make this composition:

Top: old scans, newer scans, Theodore Petersen's sketch. 
Bottom: pictures of original MS.

I've tried to process the images I got from you, but, as I suspected, the models I had for TIFFs are basically useless here. These models attempt to capture statistical variation in neighboring pixels, potentially extracting information not visible to the naked eye. This kind of information is completely lost when images are compressed into JPEGs, that's exactly the way JPEGs compress images - by removing variations that are hard to perceive with the naked eye.

Just for the reference, two of your images processed by linear regression specifically trained on some of the ink in these images. As you can see, there is no new detail and quite poor separation of ink from paint/stains.

   

The only interesting thing is, in all the images the models consistently mark a cross/ankh/dagger like shape immediately to the right of the unknown creature as ink. I'm referring to this shape from your photos: