D'oh - I just realised that my shoes are identical - the 3rd strap was hidden by the overshoes I've been wearing!
This is what they look like after 3 years of hard use.
You will probably be surprised at how light the shoes are. I noticed immediately and found that they are about 100 g a shoe lighter than my previous pair of SPD shoes. It doesn't sound a lot but don't forget that you are having to spin that extra weight round in circles thousands of times on every ride.