On the lower half the of the page (in the drawings), I notice that five people have their hands in very specific positions:

1 - Near the brown border, about 2.5 inches up from the bottom, dark haired man with fingers splayed at forehead, middle and ring fingers together

2 - To the right and up about an inch and a half, a person with a large forehead, with a loose fist in front of their mouth

3 - Three inches to the right of him, the first of three "bored men"--white shirt, his right hand curled up against side of eye

4 - An inch and a half to his right, eyes closed, his left hand on his temple with index knuckle protruding slightly

5 - Lower right corner, man with pike, four fingers showing, no thumb visible

These do not appear to be standard numbesr or letters in American Sign Language, but they could be entire words.

I already looked--ASL for "treasure" is a fist balled up just below the chin. Sign language is pretty specific and I do not feel any of the illustrations matches that word--maybe dude #2, but it seems a bit high (it should not cover the mouth).

ASL video translator:
http://commtechlab.msu.edu/sites/aslweb/browser.htm

It's a thought...