All three species are also in Revision Des Africains
Cross referencing all pictures I found back to these(drawings but good ones)makes it seem like every thing is in order in my opinion
i know of the pics in D.Sands book 2 , that black and whitish fish ,
Can't say anything on this fish as I do not have it
Most noticable differences I have found
koensis-deeper body-short stubby humeral process when compared to the others(not all koensis pictures found show the same humeral)-adipose fin slightly taller than centralis but at the very back hooks under
waterloti-triangular humeral process-adipose fin slightly taller than centralis but at the very back hooks under as above
centralis-long pointy humeral process-low adipose fin which does not hook back
i've noticed there are three pics of S.Centralis over on Scotcat , though in fairness those pics look to me like S.Waterloti !
All pictures I found seem to relate to the differences mentioned above quite well...If you compare the picture of the centralis on Scotcat to waterloti or koensis you can see the difference at the very back end of the adipose fin
Hope I make some sense...wish I could explain better
Birger