I did some research on these cartridges (UMC and REM-UMC 7.65mm Brochardt) some time ago, and can shed some light on this. Firstly, up front, REM-UMC absolutely did produce cartridges specifically boxed for the 7.65mm Borchardt. I have pictures of three individual boxes, all of the same label, in my file, including one I purchased a cartridge out of for my collection. Unfortunately, the box was not for sale. I have the UMC box for 7.65mm Borchardt in my collection.
My research found a couple of interesting things. While no powder charge, velocity etc. are discussed, UMC’s original records for the the 30 Mauser and the 30 Borchardt have absolutely identical entries for every date that an entry was made, starting in 1899 and ending in 1911. The last entry for both calibers was made May 9th, 1911, and indicated that they changed from Walsrode Grey powder (I think that was a shotgun powder, judging from earlier entries), to Bullseye. Entries of August 1899 indicated that they were changing the name of both the 30 Mauser and the 30 Borchardt to “30 Cal.”
Regarding headstamps, it is an absolute myth that the Borchardt rounds from UMC were without headstamp and with a “U” marked primer cup and that the .30 Mauser rounds had the “30 Cal” headstamp with “U” primer. I have a box with specimen for 30 Mauser with the unheadstamped case, and my UMC 30 Borchardt box was filled with the headstamp “UMC 30 Cal.” I also have an early 30 Mauser box with rounds headstamped “UMC 30 Cal,” and it is possible that there were UMC 30 Borchardt rounds with no headstamp, although I don’t know how one would tell.
Regarding the change to the “30 MAUS” headstamp, it was late, not long before the change to the R-P headstamp, which also says “30 MAUS.” It is my observation, possibly wrong but only based on what I have seen, that the “R-P 30 MAUS” headstamp is not often found, so I assume the caliber was discontinued not too long after that headstamp came into being.
It is my opinion that in the case of the unheadstamp early UMC rounds, the headtamped UMC rounds and the REM-UMC rounds, it is almost impossible to tell the difference without a box label.
As long as I have the information, I might add that evidently Kynoch differentiated between the two in early headstamps by headstamping the Borchardt rounds simply with “K” at the 12 O’Clock position and “B” (standing for Birmingham, not Borchardt) at the 6 O’Clock position, whilst the 30 Mauser rounds had the same arrangement for those two letters, but added a Star at both the 9 O’Clock and 3 O’Clock positions on the headstamp. I have not been able to verify this from box labels, but a well known British collector/dealer has indicated this and we have no reason to disbelieve it. The ratio between rounds found with the two headstamps would tend to substantiate it, with the lack of stars being the scarcer.
DWM differentiated between the two rounds when using the early " * D.M. * K." headstamp by eliminating the stab bullet crimps on the Borchardt while retaining them on the Mauser round, and by loading the Borchardt rounds with a copper primer cup rather than the more common brass cups used by them on other calibers, including .30 Mauser. I do not know how they differentiated between the two when they started headstamping rounds with “DWM” and the case number. Early boxes specifically for the Borchardt round show the number 403a on the label, but I have a sealed box of .30 Mauser rounds (so designated on the label) also marked 304a. I have never opened it, and don’t know what the headstamp is, but I don’t know of anyone who has ever seen a headstamp using the DWM marking that had case number 403a on it (403 is the normal number for 7.63mm/.30 Msuser), nor any other headstamp peculiar to the .30 Borchardt.
Another myth is that DWM rounds with greased bullets are always Borchardt. We have seen, and have in our collection, greased-bullet rounds taken from 7.63mm Mauser DWM boxes, and also with the features of the 7.65 Borchardt round. The greased bullet alone will not differentiate the two calibers as produced by DWM of Germany.
Winchester made ammunition offered for use in the Borchardt pistol, but I have not seen a box label for it. Dr. G. L. Sturgess, a reknowned expert on early automatic pistols and their ammunition, in an article “Borchardts’sche Selbstlade Pistole, Part II,” that appeared in the “Gun Report” magazine, March 2001, pages 42 thru 49, says "Indeed as late as the 1930s, after the date (1931) of the Olin Industries take over, Winchester was selling 7.63 Mauser in cartons marked “for Mauser and Borchardt pistols.” In my own 45 years of collecting auto pistol ammunition, I have never seen any Winchester 7.63/7.65x25mm cartridge headstamped specifically for the Borchardt, nor any unheadstamped round of this case type identifiable as being of Winchester manufacture. Dan Shuey, in his excellent work on W.R.A.Co.-headstamped cartridges doesn’t even deal with the 7.65mm Borchardt round as a separate entity, but rather subtitles the page on 7.63 Mauser with “7.65 Borchardt casing for Mauser Model 1896 Automatic Pistol.”