You are in MGRS zone 17S. MGRS also has grid square designations after the zone, and before the easting/northing numbers. A full UTM coordinate string will only have N/S at position 2/3, while a full MGRS coordinate string will have 3 characters at that position. A MGRS string will also be 2 characters shorter than an equivalent UTM string.

Civilian UTM doesn’t handle things near the poles very well. MGRS explicitly uses the zone characters to change to a polar projection. Those other 4 characters not used for row identification actually identify polar areas (NE, NW, SE, SE semi-hemispheres).