Ahem, yes. You'd think I might have noticed that very conspicuous bolded word **Determinant**

So, the best I can do is a really ugly kludge that produces $\displaystyle \begin{array} {r|@{\hspace{9pt}}c@{\hspace{12pt}}c@{\hspace{9pt} }|} \multicolumn{3}{r@{\hspace{1pt}}}{\overbrace{}^{\t extstyle m} \overbrace{}^{\textstyle n}} \\ p\,\{&A&B\\ q\,\{&C&D \end{array}

$.

Code:

\begin{array}
{r|@{\hspace{9pt}}c@{\hspace{12pt}}c@{\hspace{9pt}}|}
\multicolumn{3}{r@{\hspace{1pt}}}{\overbrace{}^{\textstyle m}
\overbrace{}^{\textstyle n}} \\ p\,\{&A&B\\ q\,\{&C&D \end{array}

This has the drawback that it isn't well aligned vertically. If you want to put it in a formula like $\displaystyle M = \begin{array} {r|@{\hspace{9pt}}c@{\hspace{12pt}}c@{\hspace{9pt} }|} \multicolumn{3}{r@{\hspace{1pt}}}{\overbrace{}^{\t extstyle m} \overbrace{}^{\textstyle n}} \\ p\,\{&A&B\\ q\,\{&C&D \end{array}$ then you'll want to raise it by putting it into an \hbox along with a \raise command, as in

$\displaystyle M = \raise2ex\hbox{$\begin{array}{r|@{\hspace{9pt}}c@{ \hspace{12pt}}c@{\hspace{9pt}}|}\multicolumn{3}{r@ {\hspace{1pt}}}{\overbrace{}^{\textstyle m} \overbrace{}^{\textstyle n}} \\ p\,\{&A&B\\ q\,\{&C&D \end{array}$}$.