Mammoth Annual NN (# 1) - 1956?

I purchased this wonderful specimen today from another collector. It contains the following rebound comics: 

Century #7, December 1956 

Century #5, October 1956 

Century #1, July 1956 

I understand there is another copy comprised of Century #9, Century #10 and The Hundred #11 - all from 1957. 

So - is Mammoth Annual NN/#1 from 1956? or 1957? 

Or - were there two unnumbered editions, one from 1956, the other from 1957? 

And how does the UK #2 copy fit into this mystery? 

Naturally I have some ideas, theories, speculative notions… and just as naturally this will keep me awake for much of tonight... and so the rest of the story will have to keep until tomorrow. So until then… 

PS I’ve been collating a list of Gredown Horror titles over the last week or so with the help of a few friends. Just waiting on a few more updates before I put up the first draft, so it may be another couple of days.