Biber/Bibtex: One author enumerated

Question

I'm checking my university's formatting and they want something that looks like that:

[EVER90a, S. 13-19], [EVER90b, S. 98 f.], [WECK91, S. 110]

I don't have any problems with the style of [WECK91, S. 110], but when it comes to "alphabetically counting" the different pages of one author I find myself trapped. How can I put this "a, b, …" behind the Year, when I have more than one citation of one book?

Thank you.

Here You can find an MWE:

\documentclass[listof=totocnumbered, listof=nochaptergap, bibliography=totocnumbered, parskip, captions=nooneline, fontsize=11pt, twoside=semi, numbers=noenddot, open=right, draft=false, a4paper, footinclude=false, fleqn]{scrreprt}%Dokumentenklasse 
\usepackage[svgnames,table,hyperref]{xcolor}
\usepackage[T1]{fontenc}%Zeichensatzkodierung von 7bit auf 8bit 
\usepackage[utf8]{inputenc}%Zeichensatzkodierung Unicode bzw. UTF8
\usepackage[ngerman]{babel}%Silbentrennung nach neuer Rechtschreibung 
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage[pdftex]{graphicx}
\usepackage{pdfpages}
\usepackage{remreset}
\usepackage{mathrsfs}
\usepackage{icomma}%Formatiert die Kommas in Matheumgebung ohne Leerzeichen danach
\usepackage{placeins}
\usepackage{wasysym}
\usepackage{helvet}
\usepackage{tgheros}%macht griechische Großbuchstaben möglich, trotz HELVET-Paket
\usepackage{sansmath}
\usepackage{subfig}
\usepackage{eso-pic}
\usepackage{booktabs}
\usepackage{nameref}
\usepackage{ifthen}
\usepackage[left=2.5cm, right=2.5cm, bottom=2.6cm, top=2.6cm]{geometry}
\usepackage{fancyhdr}%Kopfzeilenformatierung
\usepackage{setspace}
\usepackage{calc}
\usepackage[backend=biber, style=alphabetic, isbn=false, url=false, firstinits=true, maxnames=10]{biblatex} %maxnames ist die Anzahl der Namen, die im Inhaltsverzeichnis angezeigt werden. Darüber hinaus wird mit "u.a." abgekürzt.
\usepackage{csquotes}
\usepackage{tocstyle}
\usepackage{array}
\usepackage{longtable}%Tabellen, die über einen Seitenumbruch hinaus gehen ohne abgeschnitten zu werden.
\usepackage{hhline}
\usepackage{xparse}
\usepackage{placeins}% Package für Floarbarrier um Bilder nicht über Kapitel zu verschieben
\usepackage{titlesec}% Textüberschriften anpassen
\usepackage[bookmarks, raiselinks, pageanchor, hyperindex, colorlinks, citecolor=black, linkcolor=black, urlcolor=black, filecolor=black, menucolor=black]{hyperref}


%%% Citation %%%
\newcommand{\zitat}[2][]{\nopagebreak\nolinebreak\mbox{\cite[][#1]{#2}}} 

%%% Literaturverzeichnis %%%
\addbibresource{Literature.bib}
\DeclareLabelalphaTemplate{\labelelement{\field{citekey}}}% Damit der "citekey" in der Thesis als Zitierungsformatierung verwendet wird


\begin{document}
To make a citation you have to mention an author \zitat{SCHM05}.
But if you have one citation from pages 16 and 17 \zitat[16-17]{MONT09} and another from 20 and 21 \zitat[20-21]{MONT09} the cite key should be changed.
\end{document}

This will be the bibliography:

    @book{MONT09,
    Author = {Montgomery, Douglas C.},
    Date-Added = {2017-03-15 01:45:28 +0000},
    Date-Modified = {2017-03-15 04:45:25 +0000},
    Edition = {6},
    Keywords = {SPC, Control chart,},
    Publisher = {John Wiley and Sons, Inc.},
    Title = {Introduction to statistical quality control},
    Year = {2009},
    }

@book{SCHM05,
    Author = {Schmitt, Ingo},
    Date-Added = {2017-03-09 23:26:55 +0000},
    Date-Modified = {2017-03-09 23:29:03 +0000},
    Isbn = {3-486-57907-X},
    Keywords = {{\"A}hnlichkeitsma{\ss}, Distanz, {\"A}hnlichkeit},
    Publisher = {Oldenbourg Wissenschaftsverlag GmbH, M{\"u}nchen},
    Subtitle = {Retrieval, Suchalgorithmen und Anfragebehandlung},
    Title = {{\"A}hnlichkeitssuche in Multimedia-Datenbanken},
    Year = {2005},
    }

Answer 1

In the comments I think we established that the rule 'use a different letter for different pages' stems from a misinterpretation of the rules that actually call for a disambiguation letter in case there are two entries by the same author from the same year.

\DeclareLabelalphaTemplate{\labelelement{\field{citekey}}}

Is a very ad-hoc way to achieve this the desired label format.

Use

\DeclareLabelalphaTemplate{
  \labelelement{
    \field[final]{shorthand}
      \field{label}
      \field[strwidth=4, strside=left, uppercase=true]{labelname}
  }
  \labelelement{
    \field[strwidth=2,strside=right]{year}
  }
}

with maxalphanes=1 instead.

\documentclass[12pt,a4paper]{article} 
\usepackage[backend=biber, style=alphabetic, isbn=false, url=false, firstinits=true, maxnames=10, maxalphanames=1]{biblatex}

\usepackage{filecontents}  
\begin{filecontents}{\jobname.bib}  
@book{MONT09,
  Author = {Montgomery, Douglas C.},
  Edition = {6},
  Keywords = {SPC, Control chart,},
  Publisher = {John Wiley and Sons, Inc.},
  Title = {Introduction to statistical quality control},
  Year = {2009},
}

@book{SCHM05,
  Author = {Schmitt, Ingo},
  Isbn = {3-486-57907-X},
  Keywords = {{\"A}hnlichkeitsma{\ss}, Distanz, {\"A}hnlichkeit},
  Publisher = {Oldenbourg Wissenschaftsverlag GmbH, M{\"u}nchen},
  Subtitle = {Retrieval, Suchalgorithmen und Anfragebehandlung},
  Title = {{\"A}hnlichkeitssuche in Multimedia-Datenbanken},
  Year = {2005},
}
\end{filecontents}  
\addbibresource{\jobname.bib}

\DeclareLabelalphaTemplate{
  \labelelement{
    \field[final]{shorthand}
      \field{label}
      \field[strwidth=4, strside=left, uppercase=true]{labelname}
  }
  \labelelement{
    \field[strwidth=2,strside=right]{year}
  }
}

\begin{document}
To make a citation you have to mention an author \cite{SCHM05}.
\cite[16-17]{MONT09}

\printbibliography
\end{document}

gives then

[MONT09], [SCHM05]