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"Although the basic card reader design was originally developed for the HP-71B, the first version appeared in the HP-75 computer".

This sentence appears on page 20 of the July 84 issue of HP Journal.

Can anyone explain this ?

There's nothing particularly mysterious about it. The 71B and 75 were each in development for a long time. It's my understanding that the 71B project expanded considerably beyond its original scope. It's not surprising that something developed for the 71B (possibly before the "71B" designation was even chosen) might have first shipped in a different product.

There's a long history of such occurrences in the computer industry. The transitor circuits and packaging and the core memory units IBM originally developed for their Stretch computer were first shipped as part of the 7090 computer in 1959, two years before the first Strech (7030) shipped.

Why there is the "B" in 71B? I assume that is from BASIC, but as far as the 75, the C was customary for CMOS Continuous Memory (The 75 C had BASIC too... but it evolved into the 75"D"...)

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.

And the HP71B was almost called the HP44 (another machine in the HP41 series...). There are copies of documents in the HPCC library that refer to the HP44 (and it's clearly the same as the HP71B that was actually sold).