Antidraft groups believe they have received a mighty assist from a little-noticed finding by the Selective Service System.

Selective Service, according to Director Robert E. Shuck, has concluded from a recent computer test that all the 18-year-old males in the country could be registered in five days even though there is no list on file at local draft boards.

Such speedy registration, contends the Committee Against Registration and the Draft, means that There is no need for Congress to pass bills to require young people to register at their draft boards upon reaching 18.

Selective Service Reported its finding in a letter Shuck sent Sen. William S. Cohen (R-Maine) on May 10.

"The preliminary findings" of a test of using computers to register young people who had not previously signed up "indicate that such a system is quite feasible for accomplishing the input of registrant data quickly and efficiently in an emergency situation as well as during continuous registration," Shuck wrote.

Shuck added that if Congress approved the extra $1.7 million Selective Service is requesting fro fiscal 1979 as well as its regular budget request of $9.8 million for fiscal 1980, Selective Service would be able to gear up to process draftees so they could be sent to training camp within 30 days of a Pentagon call.