The Salamis Tablet
Counting boards are the earliest known form of a counting device (excluding fingers and other very simple methods). They were made from natural material such as stone or wood, and the actual counting was done on the board with beads, pebbles, or even colored sand. Not many of these boards survive today because of the perishable materials used in their construction. The oldest known example of a counting board was discovered on the Greek Island of island of Salamis in 1899.
It is thought to have been used by the Babylonians in about 300 B.C. and is more of a gaming board rather than a calculating device. The gaming boards used by ancient cultures such as the Babylonians and the Romans are thought to be the precursors of the Abacus. The Salamis tablet is approximately 150 x 75 x 4.5 cm and is made of marble. Parallel grooves and Greek symbols are carved into it.
The Roman Abacus-Inference of Zero and negative numbers
When using a counting board or abacus the rows or columns often represent nothing, or zero. Since the Romans used Roman numerals to record results, and since Roman numerals were all postive, there was no need for a zero notation. But the Romans clearly knew the concept of zero occuring in any place value, row or column.
It may be also possible to infer that they were familiar with the concept of a negative number as Roman merchants needed to understand and manipulate liabilities against assets and loans versus investments.