Senate Majority Leader Robert C. Byrd (D-W.Va.) caught filibustering Republicans off guard yesterday with a surprise maneuver to build support for the administration's labor law revision bill by offering an amendment intended to reassure small businesses.

In a further move to marshal support for cloture votes to end the filibuster after the coming week's Memorial Day recess, Byrd indicated that more concessions may be made.

"If cloture is invoked," Byrd said, "the Senate can proceed to consider substantive amendments which will result in a better bill."

Sen. Harrison A. Williams (D-N.J.), the bill's chief sponsor, said the amendments may involve some of the measure's most controversial provisions, including deadlines for union representation elections, union access to company property, and penalties for bargaining delays.

The amendment Byrd introduced yesterday would prohibit the National Labor Relations Board from expanding its jurisdiction over small businesses, about 75 percent of which are now exempt from national labor laws, according to Byrd.

In earlier debate, filibuster leader Orrin Hatch (R-Utah) warned that NLRB might expand its jurisdiction at any time, thus exposing more small businesses to restrictions envisioned by the bill.

Byrd said his proposed amendment "answers, directly and fully, the charge that . . . NLRB will proceed to extend its jurisdiction to cover hundreds of thousands of small employers."

Hatch said the bill is still objectionable, and "no one should be deceived by this ploy."

Some of the most effective lobbying against the bill has come from small businessmen who fear unionization. Both the Chamber of Commerce of the United States and the Small Business Legislative Council termed the Byrd amendment insufficient protection for small business.

The AFL-CIO said it though the amendment was unnecessary but would go along - essential the same position as that taken by Labor Secretary Ray Marshall, who met with Byrd before the amendment was introduced.

It takes 60 votes to break a filibuster, and both sides are claiming enough. An administration source said he was confident Byrd would have enough votes to end debate by June 15, but opponents have pledged to extend the fight by introducing hundreds of amendments.