Change alphabet for sorting in biblatex for same year

I take MWE from here Sort bibliography with same author and same year (biblatex)

\documentclass{article}
\usepackage{filecontents}
\usepackage[style=authoryear,sorting=nyt]{biblatex}
\begin{filecontents}{\jobname.bib}
@BOOK{lennon1972a,
AUTHOR = "John Lennon",
TITLE = "My music",
YEAR = "1972"}
@BOOK{lennon1972b,
AUTHOR = "John Lennon",
TITLE = "More of my music",
YEAR = "1972"}
\end{filecontents}
\begin{document}
\cites{lennon1972a}{lennon1972b}
\printbibliography
\end{document}


Is it possible to change 'a'...'b' to another alphabet for a given language alphabet? Let Say to Roman numbering

EDITED:

According to answer below I changed code as folllow:

\documentclass{article}
\usepackage{filecontents}
\usepackage{fontspec}
\defaultfontfeatures{Ligatures=TeX}
\usepackage[russian]{babel}
\usepackage[style=authoryear,sorting=nyt]{biblatex}
\setmainfont{Dejavu Serif}
\begin{filecontents}{\jobname.bib}
@BOOK{lennon1972a,
AUTHOR = "John Lennon",
TITLE = "My music",
YEAR = "1972"}
@BOOK{lennon1972b,
AUTHOR = "John Lennon",
TITLE = "More of my music",
YEAR = "1972"}
\end{filecontents}
\DeclareFieldFormat{extrayear}{% = the 'a' in 'Jones 1995a'
\iffieldnums{labelyear}
{\Asbuk{#1}}
{\mkbibparens{\Asbuk{#1}}}}
\usepackage{enumitem}
% Some other packages, not needed for MWE
\renewcommand{\theenumi}{(\Asbuk{enumi})}
\renewcommand{\labelenumi}{\Asbuk{enumi})}
\begin{document}
\begin{enumerate}
\item Item A
\item Item B
\end{enumerate}
\cites{lennon1972a}{lennon1972b}
\printbibliography
\end{document}


As you can see Russian letters are outputed for enumitems, but there is an error for biblatex part

The extrayear field is just a number that you can format as you want.

The default definition (biblatex.def) is

\DeclareFieldFormat{extrayear}{% = the 'a' in 'Jones 1995a'
\iffieldnums{labelyear}
{\mknumalph{#1}}
{\mkbibparens{\mknumalph{#1}}}}


For Roman numerals you can make that read

\DeclareFieldFormat{extrayear}{% = the 'a' in 'Jones 1995a'
\iffieldnums{labelyear}
{\Rn{#1}}
{\mkbibparens{\Rn{#1}}}}


or \RN for uppercase Roman numerals instead of \Rn.

For Russian labels as printed by \asbuk we need \russian@alph

\makeatletter
\DeclareFieldFormat{extrayear}{% = the 'a' in 'Jones 1995a'
\iffieldnums{labelyear}
{\russian@alph{#1}}
{\mkbibparens{\russian@alph{#1}}}}
\makeatother


(note the \makeatletter/\makeatother pair).

MWE

\documentclass{article}
\usepackage{filecontents}
\usepackage[style=authoryear,sorting=nyt]{biblatex}
\begin{filecontents}{\jobname.bib}
@BOOK{lennon1972a,
AUTHOR = "John Lennon",
TITLE = "My music",
YEAR = "1972"}
@BOOK{lennon1972b,
AUTHOR = "John Lennon",
TITLE = "More of my music",
YEAR = "1972"}
\end{filecontents}

\DeclareFieldFormat{extrayear}{% = the 'a' in 'Jones 1995a'
\iffieldnums{labelyear}
{\Rn{#1}}
{\mkbibparens{\Rn{#1}}}}

\begin{document}
\cites{lennon1972a}{lennon1972b}
\printbibliography
\end{document}

• so just \Roman does not work right? see udated question – Levan Shoshiashvili Jan 26 '16 at 17:26
• @LevanShoshiashvili As far as I can see, \Roman takes a counter (name) as argument, but we need a macro that can directly take a number. That is why I used \RN. – moewe Jan 26 '16 at 17:30
• @LevanShoshiashvili One just needs to identify the function that actually creates the output from the numbers, in this case it is \russian@alph (or \russian@Alph). Try \makeatletter \DeclareFieldFormat{extrayear}{% = the 'a' in 'Jones 1995a' \iffieldnums{labelyear} {\russian@alph{#1}} {\mkbibparens{\russian@alph{#1}}}} \makeatother – moewe Jan 26 '16 at 17:35
• Thank you very much...That works...How to define something like '\romannumeral' is different question....Can you give direction for this? – Levan Shoshiashvili Jan 26 '16 at 17:46
• @LevanShoshiashvili I have added the comment to the answer. Those commands are pretty much straightforward, you might be able to find something on this site. (If not, feel free to ask a new question.) Or just have a look at the definition in russian.ldf. – moewe Jan 26 '16 at 17:49