For what it’s worth, the numbers you note as being printed on p. 214 of the Doubleday issues are informally referred to as “gutter codes”, and indicate the week of the relevant calendar year the book was printed in. So, the “31” in the first printing indicates that it was printed in week 31 of 1958 while the “46” of the second printing would indicate week 46 of the same year. Doubleday started this practice in mid-1958. A good description of the gutter codes can be found at the Internet Speculative Fiction Database’s wiki (http://www.isfdb.org/wiki/index.php/Gutter_Codes).

Some paperback publishers did something similar by having a printing date (usually of the form mm-yy) on the last story page of the book. With respect to Nabokov’s books, two of the publishers that did this at one time or another are Fawcett and Popular Library, though I can’t off-hand say if this applies to any of their Nabokov titles.