FYI, I picked up a Casio "fx-260Solar" the other day.
Yeah, it has rubber keys with silk-screened legends, one memory, and no RPN or programmability . . . Not even base conversions, so why bother?
Well, it is a tidy little unit -- about 2.6" x 4.9" x 0.35" (without the cover). Further, if one takes out the six screws, the thick back comes off, and all that's left is a thin, 0.2" (!) front with keyboard & display.
The back does little more than protect the electronics and push the LCD & solar cell against the front, so, with a little double-stick foam, one could attach this thing just about anywhere (inside a binder, on a checkbook, to a cell phone . . . )
Its size just might make it the right unit for any application calling for a small, thin, inexpensive, attached scientific calculator.
Again, fyi . . .