The only 686 that I'm aware of without a full lug with a barrel longer than 4" are some export guns done a while back and a couple of 5" guns done nearly 10 years ago as stocking dealer exclusives.
If you really want a gun without an underlug you may have to stick with model 19 or 66, or something not mentioned yet...
A model 27 with the "tapered" barrel in 6" or 8 3/8".
