...the Japanese call it a "soroban." For what it's worth.

The one I've seen has Two rows of beads above the bar. I think most versions have only one.

For what is probably more information than anyone wants, try
http://www.hellgate.k12.mt.us/bldg1/abacus/page1.htm
for starters.

EDIT: Then again, maybe not. But for instructions on how to use the gizmo, by whichever name, try
http://www.ee.ryerson.ca:8080/~elf/abacus/leeabacus/

POST-EDIT EDIT: Do we know the origin of "gizmo" ?