I've handled slim bbl. M64's and a friend once owned one. They exist, but were dropped fairly soon after production began, because most people wanted the heavy bbl.
These early M64's had stainless action parts, which were rumored to gall a bit and were softer than carbon parts and supposedly wore sooner. That probably affected timing sooner. However, I had one - with heavy bbl. - and had no trouble with it over a span of maybe ten years. Still, if I went looking for a stainless Smith today, I'd want one of the later ones with flash-chromed carbon internal parts.
Lone Star