Break up some fine ripe plums, and boil them in a small quantity of water till soft, adding the kernels from half of the plum seeds, after bruising them. Strain the liquid through a cloth, and to each three quarts add two pounds and a half of the best brown sugar. Boil it up, skim it, and cool it; put in a quart of brandy to every three quarts of the syrup, and bottle it for use.
The Kentucky Housewife by Lettice Bryan, 1839
Comment: Here again we see the use of the pits of a fruit, broken up, as part of the recipe. Apparently it was felt that they strengthened the flavor or otherwise provided some sort of benefit, but the call to use only half instead of all of them just adds to the mystery.
We usually recommend ignoring this advice in the case of peaches and apricots since the pits do contain a small amount of (gulp!) cyanide. However we can find no indication that plum seeds contain this in any measurable amount, so you may add them to your heart’s content with a clear conscience.