>>Hope this helps to resolve this minor slip.
>
>It can't be a slip if it is supported by a widely used
>dictionary. It is a matter of discussion or, perhaps, of a
>controversy, but by no means a slip.

And http://www.merriam-webster.com/dictionary/abacus says either is OK, but lists abaci first.

It also states: Etymology: Latin, from Greek abak-, abax, literally, slab.

So some authorities believe there is a Latin link.