You can find your device type by one of the following methods.
1. Click the Apple icon at the top left of the screen.
2. Click “About This Mac”
3. The device type will be stated within the details window as per the image below. The date range will show you the year the device was manufactured and indicate the category to select. There are two device types that range from years 2007 – 2012, to confirm what iMac you have confirm the further details such as if your mac has a black plastic back or an Aluminium back with no optical disk drive.