\input fontinst.sty
\input multislot.sty

\mathchardef\apa=0
\catcode`\_=11
\add_to\pre_second_etx_pass_hook{\mathchardef\apa=1}
\catcode`\_=\underscorecatcode

\transformfont{ptmr8r}{\reencodefont{8r}{\fromafm{ptmr8a}}}

\installfonts
   \installfamily{XX}{ptm}{}
   \installfont{ptmr5z}{ptmr8r,psyr,latin}{multislot} {XX}{ptm}{m}{n}{}
\endinstallfonts

\bye