
Green does not specify that the 12 and 12a were mixed in a sheet, rather each was produced as a sheet of 100. Now the press sheets were two hundred stamps with a top and bottom sheet of 100. Each of those 100 sheets types could have been printed on one press sheet of two hundred.
There is not more clearer information. The guru will look in his sheet stock to see if he has the 12 or 12a sheets of 100 (I want to buy one each of those anyway) and see if either is mixed.
As an aside, Rumsey's WESTPEX Sale lot 1599 was a wonderful exhibit of seal covers used only on the 24th or 25th with a WX12 and WX13.
Back on topic, as both 12 and 12a are $7.50 seals you can't go wrong buying it. I would.