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.
In making this preserve use an equal quantity of Siberian crab-apples and plums-red plums, if possible, although any kind may be used. Wash the fruit and remove the stem and blossom ends, but do not peel, although all the black spots and knots should be removed. Cover the crab-apples with a little water and stew slowly until they are soft enough to press through a sieve. Cook the plums in another vessel until tender, adding just enough water to start the juice flowing, and pass through a sieve.
Stir the two kinds of pulp together and measure, allowing a pound of sugar to each pint of pulp. Bring the pulp to a boil and add the sugar after the latter has been heated in the oven. Continue to boil slowly until a thick preserve is obtained, stirring constantly. Fill into preserve jars and seal.
If red plums and red crab-apples are used, the result will be a rich red preserve. A green preserve is made by combining Greening apples with green-gage plums, and a yellow preserve with yellow-fleshed apples and yellow-gage or honey plums. If honey plums are used, do not use quite so much sugar, as they do not cook tart like other plums, but with a sweet, honey-like juice around them.