Remove stems and cut off the tops of the peppers and remove the seeds without breaking the shells. Cut cabbage fine as in slaw and add to it the salt and mustard seeds. Mix thoroughly and place in peppers, pressing it in tightly. Place tops on pepper cases and fasten them down with toothpicks. Place them upright in stone jar and cover with cold vinegar. Place cover over jar and put away in cool place until ready to use. They may be kept for several months before using.