Motorcycle Repair: Mufflers, dyno jet kit, harley davidson sportster

Question
I just bought a 1998 Harley Davidson Sportster XL 1200S, and I have been thinking about upgrading to stage 1. I am not sure that I've got the stock mufflers because they are pretty loud. There is a marking on each muffler : HDI 6-2 0212 0271 0223, above that 65769-97. Are these stock mufflers for the XL 1200S? Can I check if some internal parts of the mufflers have been removed?
My intention is to upgrade to Santee slip-ons (should be equal to SE), install SE air cleaner and rejet the carb. I have read that the Dynojet with Thunderslide might result in some strange throttle behaviour, so I'm  convinced that normal rejetting will be better. (Without touching the needle or vacuum piston) Am I right or wrong?

Answer
The mufflers you have are not stock. You can look through them and see if the baffles have been removed.
The Dyno jet kit will work very well with your setup. Just go with the standard Dynojet kit without the thunderslide.
Good luck and happy riding
Mike