It started with 4831. Until the 1970s the only 4831 was H4831. Then IMR introduced IMR4831, which was considerably faster than H4831. This caused some confusion especially with older data that just said "4831". The idea behind the various 4350s was apparently to indicate that they were similar to IMR4350. They were not the same.
To me the worst example was Hodgdon's use of Universal Clays, International Clays, and Clays.
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