Would Nabokov have been familiar with the meaning that, according to a wikisource (in German), refers to halved nuts as meaning something like accomplished, final, perhaps "ultimate knowledge" (volendet Erkentniss)?
This would suggest a particular meaning in connection to John Shade's death (but it comes through indications given by Kinbote, only)
 
Added two details of paintings in Amsterdam, by DIJCK, Floris Claesz van
Dutch painter (b. 1575, Haarlem, d. 1651, Haarlem)
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