I presume you are looking in Standard Catalog of Smith & Wesson.
In 1978 they started at 2D80,001 to 2D99,999 and then to 3D00,001 to 3D99,999.
Then in 1979 they started at 4D00,001, to 4D99,999, 5D00,001 to 5D99,999, and 6D00,001 to 6D10,000.
And so on, but they don't show it that way.