K-frame 38 Special cylinders are usually shorter than K-frame 357 magnum cylinders. Putting a 38 Special cylinder into a 357 frame would require fairly extensive shortening of the barrel shoulder, maybe re-thread the shank, and then shorten the ejector rod. Lots of work for little to no benefit. Since you are looking at a custom built PPC revolver, it is possible that the gunsmith replaced the cylinder with a dedicated 38 Special cylinder. If not, most 357 and 38 Special revolvers are equally accurate with 38 Special ammo.

__________________
VCDL, GOA, NRA