750d already existed in the same French sizing system that gave us 650b 700c etc: "In the French system, the first number is the nominal outside diameter in mm, followed by a letter code for the width: "A" is narrow, "D" is wide. The letter codes no longer correspond to the tire width, since narrow tires are often made for rim sizes that originally took wide tires; for example, 700 C was originally a wide size, but now is available in very narrow widths, with actual outside diameters as small as 660 mm."
https://www.sheldonbrown.com/tire-sizing.html It was supposed to be replaced...