Missing letter identification in bibliography for two papers with same author and year. (Natbib+BibTex+makebst)

Question

I've made a custom .bst file, using makebst. When using Natbib, I get for two papers in the same year and author, a letter identifying the two papers as "a" and "b" in the citation. Sadly, no letter is added in the bibliography. If I change my bibliographystyle to plainnat, I see the letters. Hope someone can help me...

\documentclass{article}
\usepackage{natbib}
\usepackage{filecontents}
\begin{filecontents*}{\jobname.bib}
@misc{Hansen2015a,
author = {Hansen, Jens},
title = {Title One},
year = {2015}
}
@misc{Hansen2015b,
author = {Hansen, Jens},
title = {Title Two},
year = {2015}
}
\end{filecontents*}
\begin{document}
\citep{Hansen2015a} and~\citep{Hansen2015b} or \citep{Hansen2015a,Hansen2015b}.
\bibliographystyle{bibstyle}
%\bibliographystyle{plainnat}
\bibliography{\jobname}
\end{document}

BST file

Custom bst-style:

PlainNat Style:

Answer 1

The issue was related to extra.label, which was missing in the original bst-file. Running the makebst again and copying the code-snip from the new bst-file works for me now.

Old code:

FUNCTION {format.date}
{
  month "month" bibinfo.check
  duplicate$ empty$
  year  "year"  bibinfo.check duplicate$ empty$
    { swap$ 'skip$
        { "there's a month but no year in " cite$ * warning$ }
      if$
      *
    }
    { swap$ 'skip$
        {
          swap$
          " " * swap$
        }
      if$
      *
    }
  if$
}

New bst-code for FUNCTION {format.date}

FUNCTION {format.date}
{ year "year" bibinfo.check duplicate$ empty$
    {
    }
    'skip$
  if$
  month "month" bibinfo.check duplicate$ empty$
    'skip$
    {
      " " * swap$
    }
  if$
  *
  extra.label *
  before.all 'output.state :=
  ", " swap$ *
}