Did you know every NGK spark plug has both a plug number and a stock number? Only the plug number is labeled on the spark plug, while both are printed on the box. The plug number is used to decode the spark plug and the stock number is used by our sales associates to aid in stock management, as alphanumeric numbers are harder to inventory. The stock number is a random 3-, 4- or 5-digit number and has no relation to heat range or plug type
Here is the decoding chart.
This video here by NGK explains what each number and letter mean in the NGK Part Number.