At418 model

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.

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.