At418 model

Does the computer not start? if it runs, "About this Mac..." will show its SUB-MODEL, what I suspect you need,

AT418 is not a valid model number. It should be two letters then four digits. Look closely to see if the "T" is actually a "1."

A1418 is a 21.5-inch Late 2012 iMac. There is no A 2418 that i can find.

If the computer does not boot, then you can get the exact model by entering the system serial number here:

Check Your Service and Support Coverage - Apple Support

The system serial number is usually found on the bottom of the computer. For an iMac, the serial number will be on the bottom of the stand.