The dimples are there for the little pegs on the front of the cylinder. Once the cylinder is turned slightly those pegs clamp the key in the keyway. They can either be sprung ball bearings, or two little copper dolphin shapped wedges.
The amount of holes also indicate the key control level. I had some with 3 dimples which indicated them at 'ruby' level. The highest IIRC is Diamond and requires the keys to be sent off for cutting, not even abloy affiliates can cut them. I might be a bit inaccurate, but you're best off reading Han's paper on the Abloy & it's history
what a beast
