Chiefly... to match the frame shape/size, except in a lightweight model, what .357 revolver (any brand), would be exactly comparable to the Smith & Wesson Model 15-3 .38 Spl. 2" barrel?nvonjvila
One small correction, The 2.5" M19/66 came with Magnas, not target grips, same as the M15The closest thing that I know of to a Model 15, is a Model 19. Rather that's as light as you want or not, only you can tell. I've owned both over the years and the Model 15 does feel and handle "lighter." The scales say it's not much, but the feel is totally different. The Model 15 has a tapered barrel, a lighter rib, and a shorter cylinder. The Model 19 has a no taper barrel, heavier rib, and a longer cylinder. Also the 19 usually has target grips, vs the Magna's found on most Model 15's. Those things don't sound like a lot of weight, but they do add up, especially since most of the difference is toward the muzzle giving the gun a totally different feel. Not bad...just different.
The same applies to the Model 24 (44 Special) and the Model 29 (44 Magnum) just scaled up.
So far the 360 or 360 PD might do... nvonjvilaI owned a 386PD for a time. First, review Geezer's comment. Second, in this case it was a seven shooter which required an N frame holster. I find a 3 inch 65 with the newer Houge S&W logo RB grips works well with .357 loads. Of course it is not lightweight.
OTOH I also once owned a 60-10 that wore Herret's RB/SB grips. Although steel and a five banger the weight difference happens. Generous grips are out there. So food for thought......