Just something SiG jumped on a few years ago to offer shooters a cheaper option. Slide assemblies and mags so one can shoot .22LR with their Classic P series pistols, without having to but a seperate handgun. Availability was sparse when first introduced as customers had been asking for them for a long time. Supplies are better now and for a time SiG offered a coupon for a ".22LR Conversion Kit" at a discounted price with the purchase of a Classic P Series. The "kits" are available now by themselves as they are listed on the SiGArms website. Like anything brand new, there were some initial growing pains and minor bugs to get worked out. Those have been rectified and the supply/demand thing seems to have stabilized as well.

No personal experience so I cannot speak to those things, but I haven't read much in the way of serious problems and whatnot.