For our 400th puzzle I thought I should post a Mate in 400, but I wasn’t able to find anything suitable, so decided to divide by 10 and offer a mate in 40 from 60 years ago instead.

You’re going to have to play a lot of checks on the long diagonal, so the first few moves shouldn’t be too hard to find.

After that it’s all about going upstairs and downstairs to clear the a-file and then deliver checkmate. The quickest way to do this takes exactly 40 moves.