Citation problem: two paper with the same first author and the same year

With my bst file (as attached) , two papers in my reference list are

Author, X, Y. Buthor, and Z. Cuthor. 2014a

Author, X, Z. Cuthor and W. Duthor. 2014b

and the citations in the article body are

Author, Buthor and Cuthor (2014a)

Author, Cuthor and Duthor (2014b)

However I would like the reference list to be

Author, X, Y. Buthor, and Z. Cuthor. 2014

Author, X, Z. Cuthor and W. Duthor. 2014
and the citations in the article body to be

Author, Buthor and Cuthor (2014)

Author, Cuthor and Duthor (2014)

How could I do it?

documentclass[11pt]{article}
usepackage[longnamesfirst]{natbib}

begin{document}
......
bibliographystyle{rfs171011}
bibliography{cds_ref}
.......
end{document}

The bst file is here, in case it is needed
https://latex.org/forum/viewtopic.php?f=50&t=32406