Adding space after equation numbering

Adding space after equation numbering

How can I add space after the equation numbering?
MWE:
\documentclass{book}
\usepackage{amsmath}
\begin{document}
\begin{align}
a = b + c
\end{align}
\end{document}
So I want to move the equation number a little bit more to the left:
a = b + c (1) <-- (1)
For your interest, I have already tried:
\makeatletter
\def\tagform@#1{\maketag@@@{(#1)\hspace{2em}\unskip\@@italiccorr}}
\makeatother