Officials of the Prince George's teachers' union displayed yesterday about 3,000 postcards filled out by county teachers in support of binding arbitration to settle their contract dispute with the county school board.

The postcards, purchased by the union, were addressed to school board President Susan Bieniasz and will be mailed to her New Carrollton home today, according to union President Paul Pinsky.

The reason for the postcards, according to Pinsky, is that the school board believed the binding arbitration clause is more important to union leaders than the rank-and-file teacher.

"In five days over 3,000 teachers have written to the school board president and more are still coming," Pinsky said at a press conference. "It is not just a management issue."

Contract talks between the teachers and the school board broke down indefinitely last week after the school board rejected a binding arbitration clause for the second time. The teachers and board had already agreed that the teachers would receive no cost-of-living increase in their contract this year. The school board also offered to shorten the teacher work year from 189 to 187 days and to give a flat $186 to about 2,200 teachers who were not scheduled for longevity increases under the current contract.

That contract expires at midnight tomorrow. On Thursday, at the school board's regularly scheduled meeting, the board is expected to pass an interim set of work rules extending certain provisions of the current contract until a new one can be negotiated.

The union placed ads in local newspapers and on radio stations encouraging union members to call board members last weekend, but according to school officials no board member received more than 15 calls.