If you buy Speer 135gr SB Gold Dot bullets for handloading they only offer one type, and its not specified for .38 spl or .357 mag. The data in their loading manual also doesn't specify a different .357 135gr gold dot for .38 spl vrs .357 mag.

It makes no sense for a company to load different bullet types of the same weight in their factory loads for the two calibers and not offer both types to the hand loading customer. The logical conclusion is there is only one type of .357" diameter 135gr Gold Dot bullet being produced by Speer for use in both cartridges.

An example of a manufacturer using different type bullets for the 2 cartridges is Remington which sells their brass jacketed 125gr Golden Saber bullets to hand loaders. The 2 different types, one designated for the .357 mag and the other for the .38 spl, the .38 spl bullet having a wider opening and deeper hollow than the bullet designed for the .357 mag.