Awesome research and comments on IH history. Thank you hydroharry, Marty Gwin, Kraig McConaughey, and IHinIN for pointing me in the right direction!
Interesting that the same part number "351 371 R1" is marked on these different-era wheel weights shown by IHinIN (relatively newer wheel weight painted white with "IH Man-On-Tractor" logo and "L" mark) and Marty Gwin (relatively older wheel weight painted yellow with different font for same part number and older IH emblem). IH's engineering, manufacturing, and marketing seemed to be all right with maintaining the same part number through the years. Would you say these different-era wheel weights are physically interchangeable without modification? Makes $s and sense.
On my substantially unrestored October 1964 IH Cub Cadet 100, the front grill panel (no headlights), dashboard, steering wheel cap, and seat support spring all display the "IH Man-On-Tractor" logo. After 33 years of ownership, I understand and appreciate this logo much better now - thanks to all of you.
Marty, I was also wondering about the meaning of the "square-inside-the-circle" mark on your relatively older wheel weight. Perhaps another manufacturing plant?