Apoligize in kicking the can down the road further, but I have a 3 die set by RCBS that is marked
06 to 25 LW SU (date code C.)
The first die is a forming die using 30-06 cases that when sized, the extra poking above the die is hacksawed off, then filed smooth. Carefully chamfered, the case is now 2.030 in length, but not yet necked down to 25cal.
On to the FL die, which is adjusted down to lightly contact the #2 shellholder that reduces the still 30cal neck down to 25cal. (Imperial die wax used for both steps.)
Dimensions at each step:
After form die and hacksaw and chamfering= 2.030
After FL die= 2.038
Datum length essentially the same= 2.892
I used my standard 6in calipers and a fired/decapped 44mag case. (Poor man`s method

)
Shoulder angle appears to be the same as the 243Win, yet fireforming would yield an answer. But which chamber would that be? Who knows.
I know there were numerous versions of the 25 Souper. A call to RCBS inquiring as to the date code rendered the response that records on hand do not go that far back. In the case of these dies go, who the heck is LW?? Back in "those" days, RCBS would make a die set (in this case a 3 die set) to whomever.
Just another piece of the wildcat history puzzle yet to be discovered.
Regards,
Scott