686 7 round cylinder on 8 3/8 inch barrel

[BZ08]

I have never been able to figure this out. How come the 6 inch and shorter barrel variants of the 686 are available with a 7 round cylinder but the 8 3/8 inch version is only available with the 6 round cylinder? Would it be possible to convert a 8 3/8 inch to a 7 round?

 

[one eye joe]

BZO8, S&W will do it for around $165. including the new cylinder. A bargain IMHO........