drduc,
Don't rechamber to 9.3x74R.Make(or have made)a chambercast,to determine what you need.The 8.7 over 75 is not the nominal caliber, it is the bore diameter(not groove or bullet diameter) as measured at the proofhouse and the length of the intended case. T&S had proprietary cartridges, but they chambered for others also.If the chambercast shows a head diameter of around .473", use 9.3X74R cases. If it measures around .428",then use 9.3x72R cases( they will be a little short, but useable), or if you don't mind shortening pretty expensive cases to gain 3mm,you can buy 9.3x82R cases and use them.Check the groove diameter from the chamber cast( also slug the barrel) to determine what bullet you need.Don't just assume you can use the pretty avaliable .365-.366" bullets intended for the more modern 9.3s.There is a 3 part article in WAIDMANNSHEIL,one of the publications of the German Gun Collectors Assn. This should be avaliable from them.There is a discussion of appropriate bullets in this article.If you are not already a member of GGCA,it may benifit you to join.I will be happy to help you all I can in making ammo for your rifle.Don't be too disappointed, if you find out from all this checking, that you can use 9.3X72R Norm.factory ammo.Whatever you learn about your rifle would be worth it anyway.
Mike