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The HP-75C was actually almost introduced as the HP-75D. They changed the suffix to "C" at the last minute, and some of the publicity photos showed the "HP-75D" legend of a prototype unit. Some prototypes were marked "HP-75CX", and I think there were other designations as well.
It is not uncommon for the final name or model number of a product to be decided only weeks before product launch.
I've never heard any explanation of the "B" in "HP-71B", though "BASIC" is as good a guess as any. At various times HP seemed to adopt naming conventions, but they never stuck to them very well until the "S" for scientific, "B" for business, and "G" for graphing that they have used for the last decade or so.