Re: Nabokov left an Easter Egg for re-readers. Did you find it? Or “Plums”

JM: I was wondering if the expression about hidden plums used by V.Nabokov is an idiom in everyday English. "Pale Fire” …” is full of plums that I keep hoping somebody will find.”
I could find no example of its use in any of the places I checked.

Cakes with little bits of candied plums are childhood favorites, together with the “finger game” rhymes in German describing a plum-tree being shaken for its fruits:  “Das ist der Daumen, der schüttelt die Pflaumen (= Zeigefinger), der sammelt sie alle auf (= Mittelfinger), der trägt sie nach Haus' (= Ringfinger) und der kleine, der isst sie alle auf (= kleiner Finger).*
Years later, plums also filled one of my favorite short-stories by Katherine Mansfield (but I forgot the title: an old spinster attends a concert, overhears a young couple’s mocking comments and the lady’s Sunday outing, cum prized plum, is spoiled). I must have brought this issue up once upon a time but it’s still haunting me.
Is there a “plum” idiom in English - in the sense favored V.Nabokov?
In Russian?  I hope a sympathetic Nabler will help me to draw an image of these appetizing purple images. VN quotes that mention the flavor or the color of a plum are also welcome.   

…………………………………………………………………………………………………………………………………………..

*- Loosely associated to John Shade’s lines about his blunt fingers and hidden “plum” rhymes (in ‘thumb’ and ‘glum’):   “…certain flinching likenesses: the thumb,/Our grocer’s son; the index, lean and glum/ College astronomer Starover Blue;/ The middle fellow, a tall priest I knew; / The feminine fourth finger, an old flirt;/ And little pinky clinging to her skirt.”  
These lines, by Shade, are very strange because they seem as guileless as some verses for children. However, they inform us that  Shade’s old flirt has grown into motherhood. The lean priest must belong to his childhood but where in time does the recurrent Starover Blue fit in?