Actually I sold it about 7 years ago, only because I was more interested in the older snappers. Pre 1990. But it ran perfectly the whole time I had it. No issues. Of course I kept it in the garage and maintained it. But yes it was a great mower.
As for the model/serial numbers, I inquired to snapper as to how to identify the year a mower was built. The info they gave me was that the first digit in the s/n indicates the last digit in the year it was built and the series number at the end of the model number would narrow down the decade, basically. For example: a model 21351PS with a s/n starting with a 4 is 21", 3.5hp, series 1 (built from 1983-1987 roughly) built in 1984. P is self propelled, S is electric start. Or a 21405PD with s/n starting with a 9 is 21", 4hp, series 5 (1988-89 roughly) built in 1989. Now commercial units labeled 21500C or 21500PC were series 0 from about 1979 all the way thru 91 or 92, something like that. Series 2 and 4 were blade brake clutch models from the mid to late 80s. After 1989 the series numbers increased so I would take a guess yours is a 1998 model.