How to find your Mac's model identifer
The Mac model identifier, also referred to as the "Model ID" or 'Machine Model" is the best way to identify your Mac.
The problem with using model numbers such as A1418 is that generally refers to the shape and can extend across multiple models. It is not a specific identifier.
To find your model identifier go to the top left of your screen, click on the Apple symbol -> About this Mac -> click on the System Report button -> it should bring up the Hardware Overview page. Your model identifier is on this page (second line).