This image comes from the French book Liber Mutus Latomorum which seems to contain historical degrees. This tower has no ladder and neither appears to be the tower of Babel.
Les Plus Secret Mysteres (1820) speaks of: “The Tower where the murderers of Hiram are locked up”. Judging the three guys above, this might well be the explanation of the image above as well.
More about the book here.
Les Plus Secrets Mystères des Hautes Grades de la Maçonnerie (1766) has a:
Figure XXI. The tower in which the murderers of Hiram were incarcerated.
Because those are three, this is a likely explanation for the image above, meaning that the Mutus appears to refer to one of the “Elu” / “Elect” degrees here.
On the other hand, what if the letters actually do simply mean “Tour de Babel”, ‘Tower of Babel’?
Mutus Liber Latomorum (1765)