Yes it does go without saying you need the right change key for the lock!

After you've correctly aligned the wheels and you carefully insert the key, look carefully- there is a 'wing' like projection on the key and a gap or wasted section just behind it- the key should go in so you see the wing just disappear into the cover and the wasted part should be aligned and seated perfectly in the hole in the cover. The hole then acts as the guide for the key to pivot as you turn it 90 degrees, and the wing acts as a stop inside once turned. With a 3 wheel key the whole lot will disappear behind the cover and prevent the key turning, and serious damage is the result if it's forced. I have a change key in my collection with 3 full twists from a military base where the guy was having a bit of trouble and decided pliers were the answer... no longer recognizable as a change key, but it would make a good corkscrew for removing wine corks...