It is not impossible that in some cases Goscinny may have made some contribution to the plot and the jokes but not enough for the publishers to give him actual credit.
That makes a lot of sense - perhaps he was a contributing gag-man rather than a 'writer' in the traditional sense for these early albums? I base that thought on absolutely no evidence at all mind!
One interesting* piece of evidence is that each page of art has Morris' signature alone, not the more familiar 'Morris & Goscinny'. The upcoming
'Lucky Luke Versus Joss Jamon' is the same, although it seems that Goscinny became '[...] the appointed scenario writer of the adventures of Lucky Luke starting from the 11th album "Lucky Luke against Joss Jamon" (Dupuis, 1958)
' so perhaps the signature proves nothing.
Actually, and I'm not sure if this helps at all, the Goscinny web-site links to a PDF
that says it is a 'Reproduction of the original synopsis written by Rene Goscinny for the album: On the track of Dalton' (thank you babelfish...). It is 23 pages long which, for a book he receives no published credit on, is pretty good going!
I think I've confused myself now**...
(* using the word in its very loosest sense...)
** Yes I have. I've just found this piece
on the Goscinny site (translated via babelfish
"The signature of Goscinny appears finally at the side of that of Morris on the covers of Lucky Luke. “The Rivals of Painful Gulch” album n° 19, Dupuis, are the first album on which figure the name of Goscinny. Although Goscinny is the scenario writer of the albums n° 9 then n° 11 and of all the following, the name of Rene Goscinny will not be reproduced for a long time on the covers of albums 9 and 11 to 18."
So, answering my own question, it looks like maybe Cinebook are wrong to credit Morris alone for this album. Phew, back to hibernation for me...