It's got the secret mark so it's a 159 or a 186. The way to tell is to check the paper.
A 159 is on "hard" paper and a 186 is on "soft" paper. The easy way to tell is to hold the stamp up to a strong light. If you see a sort of "woven" or "diamond" pattern in the paper, then it's soft paper, scott # 186, if not, then it's hard paper, scott #159.
http://www.jamesdire.net/ID/idbn6c.html That link helps to identify 6c Large Banknotes.
I always liked the design of the 6c Large Banknotes myself :)