Somerton Man Code Page
Bottom Right Corner
'R'

The image on the right above is the same lower right-hand corner shot viewed vertically, showing the 'breaks' that define the letter as an R and not a letter B:
The Concealment:
This image above contains the real code; it was taken from an oblique angle, and it had a 12-watt backlight. You can see a string of letters and numbers reading from left to right along the edge of the page. The concealment is another example of 'anamorphic' writing.
In practice, in the first half of the 20th Century, anamorphic writing could be viewed using a special lens often fitted to cameras for taking 'acute' angled photographs. Aerial photography used a similar approach; aerial photographs were often taken at various angles that needed to be interpreted by ground staff using special lenses. Cameras and/or lenses were likely used to read anamorphic messages
The famous Minox spy camera had a range of optional lenses and viewfinders created specifically for the world of espionage. https://en.wikipedia.org/wiki/Minox
For those interested, here's a link to a Peta Pixel article with images of Soviet Era spy cameras:
https://petapixel.com/2021/02/10/trove-of-clever-kgb-spy-cameras-from-the-cold-war-are-up-for-sale/