I acquired a 3-die set marked "O6 TO 25 L&W SU" and wears the date code "C" stamp (1952 production). RCBS says they have no remaining record of this wildcat caliber.
What is (was) this one, and can anyone assist in the proper use?

Make a Cerro-safe cast of the sizer and take the dimensions. At least you'd know what you have. Not necessarily what the initials mean, but you could figure out the parent case, volume, bullet diameter and then calculate velocities for various bullet weights.
Regards,
HockeyPuck

Sounds like a funny way of saying 25-06, doesn't it?

It does sound like whe 25-06 when it was in the WILDCAT stage of development?Remington did not legitimize the round untill 1969.

Start with a .270 case and full length size it in your die in question. That will be a desent start and easy to do. roger

Yes, it does sound like the early 25-06...
The L&W still is puzzling tho`. At least I have a place to start now. Thanks for the input!

I'd tend to agree with the others that your first step should be to start by getting some inside dimensions on the sizing die. The suggestions of cerrosafe and sizing a case both sound like pretty good places to start. It very well could have been a pre-standardized 25-06, but that is way I think that getting some specs will be your best bet, because even if it is it wasn't STANDARDIZED and may vary a little from the commercial 25-06 Rem. Within the last year I took in an old GEW 98 Mauser barrel stamped "25 sniper" and the gun shop owner told me that it should be no problem to "open it up to a 25-06 chamber" well I didn't want to do that anyway, but it would not have been that easy. Turns out it was one of the 25-06 predicessor which was based on a full length 30-06 case with less body taper and a different shoulder than the 25-06 Rem. Just goes to show you that without solid measurements you never really know what you've got!