I assume you are talking about the .40S&W... since blue foloowers were not used on the 9mm......
some say the mag bodies of the SW99 mags are not as good as the P99 mags (blueing not as bright) but to tell you the truth.... for the money you pay (half or even less of a P99 mag) I would not look twice and get the SW99 mags....
My understanding is that Mac-Gar makes the mag bodies for the P99 and SW99.... then Walther and S&W make their own follower and baseplate.....
I have not heared of any reports about problems for the SW99 mags....
10 round P99 mags had some issues with baseplates cracking....