Chief, The best reference I know on 38 Special is Otto Witts two volume set, headstamp guide to 38 Special and 357 Mag. All I can do is pass on what he says.
[i]U.S. 38 S&W. SPECIAL.
The U.S. headstamp you illustrate was used prior to 1936 and occurs in a number of styles with variations in letter size and a variation that lacks the period after the word “SPECIAL”. These are known with plain primers as well as both the US and US intertwined primers. There is even a variation known with a “U” primer made by UMC.
U.S.C.Co. 38 S&W SP’L
He says these likely date from the 1930s. A very similar similar headstamp (U.S.C.Co. 357 S. & W.) exists on a 357 Magnum cartridge and this load was introduced in 1935. 38 Special cartridges with this headstamp are loaded with all three types of primers also (plain, US and US intertwined). Note that the U.S.C.Co 38 COLT.SP’L. headstamp also exists with all three types of primers.[/i]
The 38 S&W Special was first made by UMC in 1899 and it is probable that USCCo was making it shortly thereafter. The only USCCO catalog I have is from 1917 and the 38 Special cartridge illustrated has the same headstamp as the one you illustrate in your post, so it could date as early as 1917 or even earlier. This catalog also illustrates all three styles of primers mentioned above and it is clear that all three were in use at the same time so they are no help to you. The price list in this catalog implies that the US primer indicates Black Powder or Lesmok loads while the US intertwined primer indicated the use of smokeless powder.
Some cartridges in the same catalog illustrate cartridges (like the 38 Short Colt) with a USCCO headstamp so the transition between headstamps must have started sometime before 1917 but it isn’t clear how long it took to go through the transition.
This same catalog also has a supplement that appears to date from about 1919 which lists for the first time 9mm Luger ammunition. The 9mm Luger is only found with the USCCO headstamp which would strongly imply that the US headstamp was dropped sometime before 1919.
From the information above, my guess, and it is just a guess, is that the US headstamp was discontinued pretty early in WWI but cases continued to be made with the US headstamp until the Bunters wore out. Just because a headstamp appears in a catalog doesn’t mean it was still being made, or in some catalogs, that it was ever made
There is some more info in the thread https://forum.cartridgecollectors.org/t/9mm/8057/1 where John and Gary spend a lot of time correcting a post I made that was full of errors dating a USCCO 9mm Luger cartridge. It provides a pretty good rundown on the various smokeless primer types used by USCCO.
Hope this helps,