I have read advice from others over the years and it seems a lot of 'old timers' used .30-'06 to make their 9.5X57 after opening the necks with three expanders of graduated sizes so's not to overstress the brass.

I found loading fresh .35 Whelen was a breeze through RCBS dies, sizing in a single step. That was without having drawn them straight, however, and the shoulders were a wee bit narrow on the unfired rounds as .35 Whelen tapers inward along its length moreso than does 9.5X57.

I've had no signs of failure after having used that brass for at least four loadings, but there could be unseen weakness near the base as the shoulders 'squared up' a tad when fired and the cases expanded outward a bit along their length.

With the slight (tiny?) shoulder of the 9.5X57, you'll not want to be lower or narrower at the shoulder than these.

L to R: New reload from Reed's, one of my unfired handloads from .35 Whelen loaded with RCBS dies (after trimming length), original (1926 coded) DWM531, fired brass from my .35 Whelen based reload.

Another idea is to locate some of these to have on hand for direct comparison: