In those early years, Dan Wesson guns were going through constant refinements...some for functionality & some for aesthetics. Dan listened intently to feedback regarding the guns, in an effort for improvement. The big knobby barrel nut was always a topic of discussion in the gun rags, so with the new 1st generation Model 15, along came a smoother barrel nut.
While that was a slight improvement, it was still considered unsightly & shortly afterward the barrel nut was moved inward, flush with the frame. THIS is the type that our thread starter (Luger89) has in his possession...
Because of the additional cost at the time, nickel guns are not near as common as the blued guns.
It's still a gray area to me, as to where the Model 15 "no dash" leaves off & the 15-1 begins...as of now, I assume that the gun with the older style sights & flush barrel nuts are part of the "no dash" family. A little later with the 15-1, they changed the sights to a smaller style & improved the ejector rod, by putting a mushroom head on it.
While I do not yet own a Model 15 like Luger89's, I do own a Model 14, of the same era.
BTW, all the grips on these guns are directly interchangeable with the later Model 15-2 guns...to help dispel any myths.