Cook prunes in boiling water 5 minutes; drain.
Cut tenderloins lengthwise almost in half.
Sprinkle cut sides with salt and pepper.
Place half each of the prunes and apple down center of one side of each tenderloin; cover with the other side.
Fasten with metal skewers; lace with string.
Place on rack in shallow roasting pan.
Roast in 325° oven until done, 45 to 60 minutes.
Remove pork to warm platter; keep warm.
Add 3/4 cup water to roasting pan; stir to loosen brown particles.
Pour into 1-quart saucepan; heat to boiling.
Shake 1/4 cup water and the flour in tightly covered container; stir gradually into drippings.
Heat to boiling, stirring constantly.
Add 1/2 teaspoon salt and 1/8 teaspoon pepper.
Boil and stir 1 minute.
Cut pork into slices; serve with gravy.