> It’s hard to say without knowing what the characters are. For the ZX80, the missing characters include five block elements (top and bottom halfs of MEDIUM SHADE, as well as their inverse counterparts), and inverse/negative squared variants of European digits and the following symbols: " £ $ : ? ( ) - + * / = < > ; , . Negative squared digits may be unifiable with negative circled digits.
ATASCII includes inverse variants of box drawing characters. I have to check whether some other pictographs are unifiable with existing characters. PETSCII includes some box drawings and vertical scan lines that are probably not unifiable. Atari ST includes two simple pictographs that were used as graphical UI elements. They look like a negative, low diagonal stroke and a negative diamond respectively. It also has six characters that together form logos which I wasn’t going to propose. TI calculators include a single character for a superscript minus 1. I don’t have a lot of information available about this set at the moment.