Hi Nyck,
I have the second ballade by Renié, but I read you got it already, but up until now there’s no way to retrieve the Concertino op.157 by Oberthur.
It seems quite odd as it is cited as a required piece in more than one occasions for the First Prize in Paris conservatoire at the beginning of the 20th century.
I couldn’t find in Utah (IHA) so I wonder how you managed to find it there. I’m very curious to get my hands on a copy, possibily with the orchestral score/parts. Nobody has ever recorded it apparently.