I guessed what was going on quite early - echoes of one part of the magnificent Listener 4000.
The grid fill did not require any specialist knowledge but I thought the final part of the end game was a tad unfair.
I can only find one web page that contains all the thematic orders including the final one. It is a pdf - mentioned in an earlier post.
To get to it I had to trawl the internet with complete list of all 6 thematic names as search terms. I dont think many would complete the final step without google.