Few days ago, this poster caught my sight:
I mean, that thickening can be seen on the key above its tooth is very typical here. Every skeleton key I had seen, was peculiar in the same way. To give you an idea of that prevalence, here are my keys below:
I'm not sure if this is indeed an English tradition (I didn't manage to find an unequivocal proof), but all my keys in Russia had been shaped ordinary.
As far as I understand, this thickening prevents a key from warping in a keyhole.