From The Bristol Board tumblr weblog:

From the auction listing: 
“Here is a great story to go with this great piece of art! At the 2014 Big Wow Comicfest Convention in San Jose, CA, there was an event where different comic artists would ink a page from blue lines of Neal Adams’ 1968 pencils for an unused page from Strange Adventures #214. The artwork would then be auctioned off at the show. Bill Sienkiewicz participated, however did not finish in time. So this piece was auctioned off at the 2015 show. Sienkiewicz lightboxed the Adams art onto a new sheet of 11" x 17" Bristol board, where he penciled and inked it himself. He signed in the lower section of the board. And that is how two fantastic creators collaborated on the same piece of art, 46 years apart!”