Paul, I have never seen any label revealing the true manufacturer of any of these “MR” headstamps:
7.9 x 57:
MR 7,9: Woodin believes “it was likely made by Manurhin”.
MR 1933: Reported by Alessio Grimaldi to be from Egypt. Also found in Spain.
MR 1934: Found in FNP (Palencia, Spain) after the Civil War.
MR 1935: Some Spanish collectors report that the date is false and that it was made for the Republican Army. Others believe that the date is correct and made by Manurhin.
7.65 x 54:
MR 1935: Also reported by some to have a false date and made for the Republican Army. I don’t believe that this is correct because I’ve found loaded rounds and fired cases in Bolivia.
MR 36 (serif): I have not read about any speculation for this one.
There are many coincidences between the cartridges used during the Bolivian-Paraguayan Chaco War (1932-35) and the Spanish Civil War (1936-1939). Some 7.65 x 54 cartridges are attributed to Spain but were already in use during the Southamerican conflict.