Hmm ... I *think* that the 'C' in the model number indicates
that it is one of the early ones based on CMOS technology, instead of
the NMOS used in the earlier ones -- 35, 45, 55, 65, 67, etc. I believe
that there was a 25C as well, a smaller version of the form factor of
the 35 et all, but with CMOS technology.

Those earlier ones *were* battery hungry. The 15C lasts a very
long time, except when a piece of heavy equipment holds some buttons
depressed for a month or two. :-)

Enjoy,
DoN.

I wouldn't bet my life on it, but I recall reading that the 'C' in all
those HP calculator model numbers meant they had continuous memory...
something really cuting edge at the time. I think the 25-C might have
been the first with it.

Much later they dropped the 'C', but all HP's (that I know of anyway)
still have the continuous memory feature.

Erik