Looking further, it seems like it could have even been the M-800. All three (M-800, M-810, M-811) have nearly identical bodies, except for the buttons in the top row where the power button is. The M-811 appears to be the only one with the option of a case. Here's a picture of various versions of all three:
http://www.casio-calculator.com/Museum/ ... -mini.html I've rewatched the film footage. The scene with this calculator begins about 15 minutes into the movie and there is no instance where we see the front of the calculator.
That being said, presuming the character Egon would have needed to power on the calculator before using it, the first place he puts his finger down is on the left side of the calculator. This would make the M-810 the most "realistic" fit within the story with its power button being on the left, even if this can never be proved for certain and the actual prop still could easily have been the M-800 or M-811 (for all we know, Harold Ramis may have been pushing random buttons).
Here's the screenshot I took of the first time Egon puts his finger on the calculator:
http://imgur.com/gallery/wCvox