The battery from my iPhone 4S does not have any screws!

I just purchased a screwdriver so that I can remove the back plate to see if I can fix the sunken down power button.

I was curious when I saw the battery, so I did a search for the steps to remove the battery in case I need to do it in the future.

Every article that I've read stated that there are two screws to be removed. However, mine doesn't have any screws at all!

1. This is the first time I've removed the back plate.
2. This phone was purchased from a major local telco about 18 months ago.
3. It is not a refurbished set; the phone was never sent to the telco for replacement.
4. The phone was not sent to any third party services for repairment.

Here's an image to explain what I'm talking about:




Here's an image of where the screws were supposed to be:



What's going on?