The Wilson stem for 30 cal is same that would be use for all 30 cal seater and I would guess same for RCBS.
Mine are all different across makes and have a small spread within a make for a given caliber.
I agree the Wilson stem is different as outside dia is larger that what RCBS would use as RCBS is attached to threads and non movable.
My Bonanza, RCBS and Wilson seater dies' stems all have threads and are moveable. 1/4-28 threads on Bonanza and RCBS.
The cavities are tapered and if you took a bullet dropped in the cavity of either there may be difference in dia were it stops forward of the ogive. It pretty easy to find that dia and some may be interested in that.
That's your idea and a good one. So you take the lead and measure all of yours then post them. Then I'll list mine for the same calibers I have after I measure them. Then all can compare them. Might want to list bullet chamber and seater stem body diameters for each to compare their diameters. There's quite a spread in clearance stems have to the bullet chamber across all makes.
I mentally mixed up my seater die's chamber diameters between 308 Win and 30 caliber magnums. My seven 30 caliber bullet seaters bullet chamber diameters at the mouth juncture:
Wilson BR chamber type .308, circa 1966;.3105"
RCBS standard .308, circa 1966; . . . . . . .3100"
RCBS standard .308, circa 1979; . . . . . . .3115"
RCBS competition .308, circa 1980's; . . . .3107"
RCBS standard .300 Win Mag, circa 1999, .3104"
RCBS standard .30-.338, circa 1967, . . . .3102"
Bonanza BR .30-.338, circa 1980; . . . . . .3093"
30 caliber bullet diameters I've measured go from .3070" to .3092".