A lot of dictionaries give Stook as having the same meaning as shook & shock with very few mentions of the quantity of sheaves involved. The compact Oxford English at ask oxford.com (
http://www.askoxford.com/concise_oed/stook?vie w=uk &
http://www.askoxford.com/concise_oed/shock_2?v iew=uk), as Lancsquiz has just mentioned, does differentiate by only giving the infamous "twelve sheaves" in its definition of Shock not in Stook. It's an odd thing but its big brother, the full Oxford English Dictionary, gives no mention of "twelve sheaves" in any of its definitions!
My Dent Dictionary of Measurement gives no mention of stook at all, nor shock in connection with sheaves but does say that its a word like dozen or gross referring to a specific number of units with 1 shock = 60 (units). It the goes on to say that in relation to boards or staves that make up a cask 4 shocks = 1 ring. the entry concludes by saying that it was originally used for specific units of imported merchandise but was then applied to a heap or bundle of identical objects.
It shouldn't really be necessary to think of the connections that are likely to appear in the Herculis puzzle in order to complete this puzzle correctly (?) but, as has already been stated in a previous thread by gardenknowin, on that basis Shock would appear to be the logical (?) choice.