Jo, for me the sentinels are visible on my middle size map. If we think about the meaning of the word sentinels and how these might be displayed on a map I went looking for symbols in the key that lay at the prescribed distance. In fact they weren’t too hard to find. The first sentinel sits on the 560 line the other 2 sit at the right distance in a chain.
I even get visual confirmation from the 780 in the form of the “cocher & la rosse” once you have found them.