While surfing the Web a while ago I bookmarked this Calculator Collector
Site.
I checked it out today and it is still active.  There are links to many
collectors home pages.
Perhaps you can find some HP calculator history.....
http://www.dotpoint.com/xnumber/collectors.htm

*******
Paul B. Webster VK2BZC wrote:

> Lee Jones wrote:
>
> > I recall HP creating the market with the HP35 in fall 1972.
>
>   A bit later in this country.
>
> >> I think the HP45 was the "business" calculator.
> > No!  <shudder>
>
>   As I recall:
> HP35: Scientific calculator, about 8 memories, half of which had
> different behaviour.
> HP45: Scientific calculator.  Had clock/ stopwatch and something else,
> ?more memories.  HP35 could be kicked into "beta test" version of this
> clock mode, but lacking a crystal oscillator, this was not particularly
> useful.
> HP55: Scientific programmable.
> HP65: Scientific programmable with magnetic cards.
>
> > It was an advanced scientific/engineering calculator.  HP35 shape,
> > keyboard, and display with more functions.
>
>   I think only the mag card version was a little fatter.
>
> > I still have it and it worked until recently when the last set of
> > batteries died.  You can split the plastic battery holder; replace
> > with 3 AA ni-cads; then reglue it.
>
>   Tried that, hard to get Ni-cads of the same quality as the original.
> Battery replacement severely limited by corrosive "sliming" from Ni-Cds,
> even rotted the beautiful gold-plated spring contacts.  Therefore
> calculator becomes dependent on the (best built you've ever seen)
> transformer/ regulated power supply.
>
>   That limited by second weak point, no strain relief boot on moulded
> socket.  Still, careful dissection, re-termination and re-moulding (hot-
> melt glue perhaps?) will likely fix that sometime or other.
>
>   As a calculator?  **Superb!**  It's still what I use when I look for
> a calculator in the workroom or need to do engineering stuff.  There is
> a second-hand/ recycled T-I "Collegiate" sitting in my sock drawer in
> case I need to calculate in the bedroom but on checking now, its
> batteries are flat).  Mind you, I mostly use the Win calculator to be
> truthful.
>
>   I'm sure they *were* the standard for the space program back then, a
> quarter century ago.  I wonder how many LCD calculators last even ten
> years?
> --
>   Cheers,
>         Paul B.