One of several non-passenger types which were issued on motorcycle plates.
These have not been seen on the roads recently - more than likely no longer issued.
1960 BASEXA100 - XB-999(?)
The XF- series was reflectorized by spraying something over the plate AND over the 62 sticker.
1992 BASEXG series
The 1992 base may have started at XG100.
The plate below was photographed on the road in 2001 and is the only one known.