Many thanks to respondents, thus far. The human face is supposed to be the most recognisable of all imagery, hence its extensive use on engraved banknotes to assist with the detection of counterfeits.
I therefore fully agree with ROHUMPY about the face of the Madonna and Child having "distinct styles of engraving" and having now looked more closely at each face, I fear that a couple are quite amateurish!
So, I set about finding the statue online taken from a similar angle. (Image has been manipulated, as the statue is bronze and therefore quite dark. It was also in colour.)
You can make your own mind up as to which engraver achieved the best result for the faces.
Incidentally, the country name at foot appears to my eyes to be virtually identical on every engraving, leading me to believe that this portion may have been engraved by a letter engraver, as was often the case.
Indeed, De La Rue (at least) would never allow a single engraver to produce an entire design of a stamp or banknote, lest they were kidnapped and forced to recreate an engraving. Instead, they would use a portrait, letter and vignette engraver and "rock-in" the portions of the design into a single die.