Pickled Plums

From 1922:

Take 5 lb. ripe plums, 1 quart vinegar, 1 lb. sugar, 1 cupful of treacle, 2 oz. cloves, 3 chillies; wipe and prick the plums carefully with a fork, and place in earthenware jars.
Boil all ingredients for a quarter of an hour.
Pour the boiling liquid over the plums, and cover them at once.
Do not attempt to use them for one week, when their condition will be excellent.