Hello.
I am the author of this article about GIJoe #21 2nd printing.
Found this thread. Some great information here.
I have already done a complete break down of GIJoe comics 2nd 3rd printing ads, bullpin bulletin and cover variants.
It seems most of the 2nd printings where from the multi pack. Issues 3,4,5 all have 2nd printings but not marked as 2nd print only way is to look at the ads.
Additional, #22 was released as 1st print with BOTH a white Spidey head and black Spidey head.
I will be happy to contribute my research to the 2nd printing database.
By the way, the hunt has now found 3 that meet the theory.
http://www.generalsjoesreborn.com/2020/04/the-hunt-for-the-rarest-g-i-joe-comic-ever-begins-now/