I recently had a wrench thrown into the gears of my logic concerning pre-war Winchester .357 Magnum factory primers. A couple of years ago I picked up a tattered box with about half of the original ammo, large flat primer in a nickel plated shell. It is shown on page 2 of this thread. The first 2 pages of the thread bandy about comments regarding whether ammo is pre- or postwar depending on the size of the primer (except Western which nearly always has a small primer).
Recently I had the opportunity to add another box to my collection which was the 1935 style (no Western or Olin callout) with the nickel plated shell stamp on the top and bottom. However, the ammo had small, round primers and a headstamp of SUPER SPEED 357 MAGNUM. After acquiring it, I began to doubt the originality of the ammo in relation to the box and contacted a gentleman who has an excellent reputation in dating older ammunition and presented my findings to him.
He consulted his database and personal boxes and assured me that my ammo was within spec and, most likely, original pre-war ammo. According to his data, small round primers in the .357 Magnum cartridge began coming out of the E. Alton factory around October of 1936. The SUPER SPEED headstamp also appeared in 1936 from both factories, E. Alton and New Haven.
There are also some differences in the design of the tops/bottoms of these boxes but I have not seen any in this thread so will not confuse things any further.
Ed