No, the key is even simpler than that, totally flat, nothing epecial at the tip. I've probably got a couple up in my loft somewhere, from when I used to own the 'sony' version of that exact same stereo. (Which was ripped out of my dash without using the key, breaking the din hole

. )
I'll look for a piccy, But I swear thats one of the key slots above the electrical connector right near the top edge of the unit. There will be another slot somewhere too, need to either use two keys, or use one key, tug that side free by a few mm, and then unlock the other side.
This is the closest I can find to the keys my Sony used. These are not 'Sony originals', but they are very similar, and apparently fit a sony radio. The originals were just a handle, plus a blade which looks very like the end (the narrow part). But as you can see, its just a totally flat piece of metal, it just slides in and pushes the clips away from the locking taps on the mounting cage.
I have to say the cages tend to distort a bit, so even with two keys inserted it can still take quite a tug to free the radio, and also the wireing harness can get caught up. So be firm, but still carefull, as you will be better off if the wiring harness remains undamaged.
Good luck.