How is a particular character code marked as a slug in a font?

[rmkaplan]

Often we know it's a slug because there is no charsetinfo for its character set no matter how hard we try.

But is there a general (device-independent) way of recognizing that a particular character doesn't exist in a character set even though other codes in the character set are real?

For the display, it appears that the offsets of all such slugs match the number stored at position 256 in the OFFSETS vector. Presumably that's a pointer to the charset's single black-box bitmap. But that offset is only relevant for the display bitmap.

Do the hardcopy devices also mark the slug codes in some generic way? Funny widths or image widths?

Also, the length of all the font-metric vectors seems to be 255+3. The display offset accounts for 255+1. Does anybody know the purpose, if any, of the other 2 elements?