I'm working from the wiki page and was using the military before 1956 and the current at the top. There was a suggestion to use r but how to r... doesn't make sense, the other pre-1956 words are b,p,q,s,u and z and of the current there's only t and possibly r that are verbs. If the grid looked more like a chessboard I would go with q but I might as well go for z cos it looks stripey!