The proposal included eight diacritics: acute (´), gravis (`), circumflex (^), hacek (ˇ), macron (¯) to mark tone, a tilde (˜) to mark nasalization, a subscript dot (.) and an underline mark (_). The trema was used for some vowels.
A typewriter keyboard was proposed as well: for the additional characters the uppercase letters had to be given up. Probably for this reason the keyboard did not get used. However the proposal of the additional characters as such is valuable as it reflects the needs for writing African languages. On the other side in quite some orthographies of African languages two-letter-combinations are used for representing additional sounds.
See also Africa Alphabet
Source: A Thesaurus of African Languages - A classified and annotated inventory of the spoken languages of Africa - Michael Mann and David Dalby, International African Institute, Hans Zell Publishers, 1987