Yes, welcome aboard......
As said above, the 4" 657 seems to be the hardest to find of all the catalogued .41s S&W has made...they were only made for two years, so it is anyone's guess how many were made.
As to cast bullets and 296/H110 (same powder, different can) as long as one is looking for top end performance and a gas check bullet design is used it leaves no more leading than any other powder and is very accurate. That said I have used two plain base bullets with no more leading than any other powder.
Cast bullets I have used with H110:
Commercial cast:
Cast Performance 255 grain
Cast Performance 250 grain
Penn Bullet SSK 300 grain (plain base I believe)
Home cast:
MP Molds 265 grain TruShot Clone
Accurate Molds 41-250L 250 grain
Mountain Molds 300 grain
Lyman 240 round nose (plain base)
That said I rarely shoot H110 loads in my Smiths any more. Much prefer velocities under 1200 fps (4") in the Smiths and H110 starts in the mid-1200s and works up even with the above heavies...
Now using 200-230 grain bullets in the Smiths with 8-10 grains of Unique depending on the bullet. Velocities run from 950-1200 depending on the bullet...
If you have not seen this post before...it is everything .41...lots of good posts and links...
.41 Magnum / 41 Special Load Data Center
..and so tell us...did you score a 4"??
Bob