I think for the record, we should mention that while Randy and Guy are absolutely correct about the primer markings on early U.S.A. - manufactured smokeless-powder cartridges, that later primer markings have been used to signify other things.
Often, the primer markings indicate a non-corrosive primer, such as the “O” impressed on German RWS and Geco (Dynamit A.-G.) primers indicating they are “Sinoxid” (non-corrosive). Sometimes they identify the primer itself. Recently, letters such as “CF” and “LF” have been used on primer cups to indicate a lead free primer.
One reloading company in England, ACE in Leeds, had primer cups embossed with the outline of a Spade (the playing cards suit, not a shovel) to indicate their brand, since they used primarily Winchester cases for their loads. (IF ANYONE HAS ANY INFORMATION ON ACE, OF LEEDS, ENGLAND, ESPECIALLY A CATALOG OR PRICE SHEET, I WOULD LOVE TO GET A SCAN OF THE MATERIAL).
Remington, it would seem briefly, used an “O” impressed on primer cups sold to reloaders, to indicate that any Remington-headstamped cartridge with that primer was not one of their own factory loads. I have examples of that on pistol cartridges in my own collection.
The point is, primer markings, throughout the history of fixed ammunition, have not exclusively identified a cartridge as being loaded with smokeless powder, although that was the reason for the American ones in the era discussed.