With Synodontis vaillanti it is a
Species inquirenda in Synodontis
Not that it is not recognized but that it is such an old description without the added evidence to go along with that it more or less would be impossible to ID.
As stated Valentine is used for some hybrids and this could have been misinterpreted.
How long does it take for them to become similar to eupterus?
That is the joy of hybrids it can most often be a guess to ID and it may not turn out as one would surmise when it comes to size and behaviour.
others say it is a crossbread between S. multipunctatus and the S. decorus
That guessing game could go on forever unless @racoll has an idea through his DNA study, I would be happy they stayed smaller and enjoy them for what they are.
Birger