is a specialized TrueType font developed by Design Science (now part of Wiris, the makers of MathType). Unlike normal fonts (Times New Roman, Arial, etc.) which contain complete alphanumeric characters, MT Extra contains only 104 glyphs —and most of them are not full characters.
This can happen if MathType cannot find the correct font metrics. Try: mt extra truetype font for mathtype
: On macOS, conflicts can occur between the system’s Apple fonts and the specific version MathType needs to function. How to Install or Fix the MT Extra Font is a specialized TrueType font developed by Design