Why is this "quadrigraph" necessary?

[Bruce Korb]

AC_DEFUN([INVOKE_AG_MACROS_LAST],[
[if test X${INVOKE_AG_MACROS_LAST_done} != Xyes ; then]
  GUILE_FLAGS
  [ag_gv=$( exec 2>/dev/tty ; set -x
  gdir=$(pkg-config --cflags-only-I guile-${GUILE_EFFECTIVE_VERSION})
  gdir=$(echo ${gdir#-I})
  gdir=$(awk '/SCM_MICRO_VERSION/{ print @S|@3 }' ${gdir}/libguile/version.h)
  printf '%u%02u%03u' ${GUILE_EFFECTIVE_VERSION%\.*} 
${GUILE_EFFECTIVE_VERSION#*\.} ${gdir}
  )]

If I change that "@S|@" thingy to "$" it and the following "3" disappear.  Why?
(And, yes, this fragment only works on recent POSIX shells.)