Sounds like you are looking for a S&W mod 617+. ie. All stainless ,10 shot , K frame ( same as medium. 38/357 frame) with full underlug. You can get aftermarket mounts for them easily. Also, begining with the 617-4 in 98' they came drilled and tapped for a mount.
If you were willing to bend on teh "nmore than 6 round" requirement, an earlier mod 17 or 617 ( both 6 shot) would work nicely as well.
hth