This section is from the book "Preserving And Pickling", by Mary M. Wright. Also available from Amazon: Preserving and Pickling: Two Hundred Recipes for Preserves, Jellies, Jams, Marmalades, Pickles, Relishes and Other Good Things.
To make these pickles use green walnuts that are soft enough to prick with a needle. Let the walnuts soak two or three days in a strong brine, made by using about three cupfuls of salt to each gallon of water. Drain them and let soak in cold water until they are the right saltiness; then cover with cold vinegar and let them stand in this for several days, or weeks will be all the better. Then drain off the vinegar into a preserving kettle, and add to each half gallon of vinegar one pint of water, one cupful of brown sugar, a dozen peppercorns, a half ounce of cinnamon, cloves and mace, and two ounces of mustard seeds. Pour boiling hot over the walnuts after the spices have simmered a while in the vinegar. Let the walnuts stand in this twenty-four hours, drain off and reheat, then pour over the walnuts again.
The walnuts should be just about right for pickling in July.