The main gist of the Letter to Lancet Infectious Diseases by Colebunders and Van den Ende  is that there was a clinically compatible outbreak of disease in Bili, Democratic Republic of Congo in 1956 - 20 years earlier than the first documented and laboratory-confirmed EVD outbreak there - when the DRC was called Zaire.
The 5-week disease outbreak manifested as a fever and rash followed after 10 or so days by bleeding from the mouth and nose and some times bloody diarrhoea. Vomiting blood was a precursor to death. Those without any bleeding survived and recovered after 3-weeks.
About 80 of at least 215 people died (37%), with cases clustering among family groups. A very good survival rate for some filovirus infections. Quarantine efforts and safe burials seemed to halt the outbreak although some bodies were "recovered" by family for re-burial under more traditional circumstances. Its not clear if this process caused any new infections as it did in west Africa in 2014.
The authors of this letter conclude that the disease course, consumption of monkey meat (but not bats, despite the proximity of colonies) suggests the outbreak could have been due to a filovirus of some sort.
Unfortunately we don't know which filovirus, if a filovirus at all, was responsible since there was no laboratory testing to confirm the specific agent.
So this one remains a "could be" for now. If only there were stored samples.
- Yes, there were signs that Ebola was in west Africa, perhaps as far back as 1973...
- Filovirus epidemic in 1956 in Bili, DRC