The "4 Ages of Man" Folio

[Dana Scott]

I am also curious about the wall part, see how the semicircle is extended on this one and a mark placed on the left side. Did they just overshoot and mark it to correct, or does it mean something else?

Perhaps what you are referring to is a misaligned "rivet" (red dot)?

No, I have put white arrows on the diagram regarding what I was referring to there. Do you think the rivet misalignment is meaningful? 
And what of the pebbly triangles?
 

I would say that I think the unpainted parts of the left sleeve were left as drawn to emphasize the "sleeve piping". The triangles might just be artifacts of the paint, not sure. I agree that piping is also evident on the right sleeve, whether stitched in or just a result of curving the arms up and down. 

While the areas pointed to by the white arrows are probably meaningful, I would think that their meaning is secondary compared to the overall intent of the drawing as a whole.