2984783 is the number cast in plastic on the center grill for 1970 Challenger.
The part number itself will be in the parts book with a 4327 number followed by color such as 4327FX9 for black, 4327GB7 for blue and so on.
2984028 is the cast number for 71 center speaker grill.
The difference is in the length of the shoulder on the grill for mounting.
Picture shows the two lengths.
Top example is 71 up style , longer on front shoulders.
70 only style, short on front shoulders.
Hope this helps in answering your question.