It's proposed for those planes with more than 8,000 hrs on the airframe. I assume it is for at least most of the 400 series, because our C401 would be effected by it along with the C402. Since the 300 series is virtually the same airframe, I wouldn't doubt that those would be effected also...
No solid sources on this one, but I'm told from a guy who sounded 100% sure that its for >8,000+ 402's and 401's (at least).