It’s far more of an old-school calculator than the others, reminiscent of calculators from the 1970s than the 1980s, so was possibly the prototype of the hugely successful MG-880 that appeared in 1980.
This could also explain why there are few of them to be found these days. While their electronic innards lack a bit of the refinement of the later calculators, and are a bit rough-and-ready, it is identical in function to the other variants.
So it’s entirely possible Casio used an existing case to try out their new idea, and released a smaller batch to see how it would play out in the marketplace.
This I’ve seen in practice with the FX-880 calculator. If you ever find a Casio FX-910 calculator you’ll notice it’s almost identical to the FX-880 other than a few different button placements and minor graphics. Of course it also doesn’t have the games that are in the FX-880.
The clincher is if you look on the back of the FX-880 you’ll see the label with it’s details, very neatly done, in the recessed area on the caseback.
However, if you take a sharp blade and carefully lift off the label, underneath you’ll find an embossed set of details for the FX-910.
Unfortunately the FX-880 is as rare as hens’ teeth so unless you’re really lucky and find one, you’ll just have to take my word for it.
Back to the MG-660, playing the game is a touch more difficult than the other similar models because you have to push the keys in much further.
That said, it doesn’t take long to get up to speed, but it’s certainly a different experience to the others. Worth picking up if you ever see one for its rarity value, especially since if my theory is correct, it’s pretty much a prototype of one of Casio’s most successful game calculators of all time.