I don’t know any cartridge named “9x72R”, but there may be a common confusion about the old German proofmarks. From 1911 to 1939 German proofhouses marked neither he bullet nor the groove diameter nor the cartridge name with their marks, but the bore/land diameter of the barrel. This was measured by pushing cylindrical gages in .1 mm increments through the barrel. If an 8.8 mm cylinder passed, but an 8.9 mm one did not, the barrel was marked 8.8 mm. So you have to add two times the groove depth to that land diameter to get at the bullet diameter. “8.8 mm / 72” is a common marking found on 9.3 x 72R barrels with the deep bp-lead bullet grooves. As the 8.8 mm mark is found on smokeless 9mm barrels too, misinterpreting such a mark may have lead to the “invention” of a non-existing “9x72R” cartridge. Remember, min-max rifle bore-groove dimensions were fixed in 1926 only. Before, many undersized (by later standards) barrels were proofed if they stood firing he proof load.