Paging Bob Wright, paging Bob Wright...
As I understand it, the "dash" numbers usually signify a minor design variation, not a production run. Functionally, I doubt there is a detectable whit of difference between a 686-1 and a 686-6, unless one is a six-shot and the other a seven-shot.
There have occasionally been fairly major design variations denoted by the dash numbers, though. The one that pops into my mind is the 25-2 (.45 ACP) and the 25-5 (.45 Colt).
I am sure Bob Wright will be along presently to expand (or correct) on this. His knowledge of revolvers vastly exceeds my own...and that of anyone else I know.