Symbol 6144 from IEC 60417 - Title/Meaning: Overlay, combining long slash Function/description: To identify the key to type the character of combining long slash.
Additional information: 
This symbol corresponds to the hexadecimal code U+0336 named COMBINING LONG SLASH OVERLAY of the Unicode ISO/IEC 10646-1 when this key is entered by the dead key method.
The outlined rectangle is presented as such, or as a solid rectangle, in a colour different from the colour which otherwise is used for characters on the keyboard (usually a lighter colour or gray shade). In this case, the other parts of the symbol are to be presented in the colour which otherwise is used for characters, obscuring the rectangle where they overlap it.
Own work using: iso.org/obp/graphics/grs/b3ef1da7-4f14-45b4-9d31-cf84b97fffb5 200.png
Vectorization:  Mrmw