SERIAL NUMBERS:
for this particular 5906 example (in the OP), VAN78xx is explained as follows:
In March of 1984, Smith & Wesson began using a 3-letter serial number prefix for all of their products, including autoloaders. The 5906 was introduced in 1989. In 1994, the "VAM" prefix began to be used, along with 4 numerals. By 1997, the "UAMxxxx" numbering began. So by deduction, 1994 started with VAM0001-VAM9999, then proceeded to VAN0001-VAN9999, and so on. I don't know much about the rest of the numbers, but would assume that Smith & Wesson would go from "VAN" to "VAP" (and skip the "O" in the alphabet) because of the problem in misidentifying a letter "O" as a numeral "0" (zero). But I'm not sure of that!
MODEL NUMBERS explained:
The first two digits represent the caliber:
39 - 9mm
59 - 9mm
69 - 9mm
10 - 10mm
40 - .40 S&W
45 - .45 ACP
35 - .35 TSW
The third digit indicates the type of model:
0 - Standard Model
1 - Compact
2 - Std. with Decocker
3 - Compact with decocker
4 - Std., DA only
5 - Compact, DA only
6 - Non-Standard length barrel
7 - Non-Standard length barrel, with decocker
8 - Non-Standard length barrel, DA only
The fourth digit indicates material:
3 - Alum. frame / S.S. slide
4 - Alum. frame / steel slide
5 - Steel frame & slide
6 - Stainless frame & slide
7 - S.S. slide / steel frame (two tone)