This is something I saw about 15 years ago. The 2 calculators were in the same series as the fx100 but simpler if my memory serves me correctly. One of them might have been the fx82? Could someone give me a link to a casio collectors page so I can check the model numbers?
The 2 calcs looked exactly the same with the same number of keys etc. The only 3 differences between the 2 calcs were:
1) The price (about 20% I think)
2) The display (the LCD element on the more expensive one had 'SD' and the cheaper one 'ERR' or perhaps 'ERROR' in exactly the same place)
3) The key legend on the face plate (the more expensive one had SUM(x) SUM(x^2), Sigma(n), Sigma(n-1) etc. printed on the metal, the cheaper one was blank in these places.
The operation went something like: Key sequence to enable stats mode (on the cheaper one it displayed 'ERR', the user then probably pressed AC and the 'ERR' anunciator went away). On the more expensive one 'SD' lit up and the user could then enter data and calculate AVG(x) etc.
HOWEVER if data was entered on the cheaper model when it displayed 'ERR' all the stat functions worked by using the same keys as the more expensive one.
The guy who showed me this had scratched into the metal face plate the key symbols printed on the more expensive model so he could use stats on his cheaper calc.