In her colonial period, Portugal had two territories using ‘M’ and two, ‘G’.
It may be that to differentiate, one of each (Macao and Goa) used a full hyphen set of two dashes (in the style of mainland Portugal) and the others (Madeira and Guinea), one dash only following the code letter. Proof needed
Every one of the single-letter Portuguese territories was – and remains – amongst the rarest of sightings and even photographs or plates are unknown. (Unless YOU know otherwise????)