Mock Olives Made From Plums

Plums prepared by this recipe taste just like real olives.
Two quarts of green plums, one and one-half tablespoons mustard seed, two and one-half tablespoons of salt, two quarts of vinegar.
Place plums in stone jar with mustard seed. Turn vinegar in kettle and bring to a boil, then pour over plums boiling hot. Cover closely. Next day drain off the vinegar, bring again to a boil, pour over plums. When cold put them in olive bottles.