Susan E. Sweeney: In response to Jansy's inquiry, I assume that Nabokov has in mind the English nursery rhyme "Little Jack Horner," in which the title character pulls out a hidden plum from a pie and then congratulates himself.
D.B.Johnson: Susan’s answer re Jansy’s  query is  certainly on target. I would add that “plum” as in “plum good”  or “plum smart” was quite common in my semi-rural Indiana  childhood---possibly echoing its use in the old English childrens’ rhyme. 

Jansy Mello: Great indications and a stimulus-guide for probing further. Thank you.

Since V.Nabokov  mentioned at least one other English nursery rhyme (“Who killed Cock Robin” ) in RLSK, a novel that mocks the detective story genre and, in “Lolita,” he mentions  Agatha Christie, who favored children’s rhymes for her titles and quite often, as clues about the plot (“One, two buckle my shoe,” ”Hickory Dickory Death,” “Five little Pigs,” “A Pocketful of Rye,”) I was curious about the innocent wording of “Little Jack Horner” which I heard of for the first time today.

Indeed, just like in various other instances, the lines (and their variants)are related to historical and political events from the Sixteenth Century onwards. However, although the nursery rhyme probably originated the idiom “have a finger  in the pie,” these extensions were  certainly not intended by V.Nabokov.

The context in which V.Nabokov employed his hidden plum is closer to the more happy and sensuous meaning of the rhyme and to a childhood’s “plum good” recollection!
In brief, the temptation towards what will turn out to be an “over interpretation” must be resisted.