Curve Integral typeset problem

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{equation}
\Psi(2)-\Psi(1)={\displaystyle
\int_{\underset{\operatorname{{po}\,
\Gamma}}{(1)}}^{(2)}}
(\nabla\Psi)\cdot \mathrm{d}{\vec{s}}
\end{equation}
\end{document}

\documentclass[preview,border=12pt]{standalone}
\usepackage{mathtools}
\DeclareMathOperator{\po}{po}

\begin{document}
\abovedisplayskip=0pt\relax% don't use this line in your production
\[
\Psi(2)-\Psi(1) = \int_{\substack{(1)\\\po\Gamma}}^{(2)}(\nabla\Psi)\cdot \textrm{d}\vec{s}
\]
\end{document}

Output

Compare with the scanned image on the question.

Is there any significant difference? I don't think so.

TeX Primitives are still cool:

\documentclass{article}

\begin{document}
  \begin{equation}\label{fyz:eq_fey_curveint1}
    \Psi(2)-\Psi(1) = 
      \displaystyle\int_{(1)\atop\text{po}\,\Gamma}^{(2)}(\nabla\Psi)\cdot\,\mathrm{d}\vec{s}
  \end{equation}
\end{document}

Edit 1: There is one thing that I would adjust. Instead of the \text{po}\,\Gamma, I would rather do

\usepackage{amsmath}
\DeclareMathOperator{\po}{po}

and replace it by \po\Gamma.

Edit 2: For size correction of the subscript you have to do _{\scriptstyle (1)\atop\scriptstyle\po\Gamma}.