To better understand how the Leonardo was damaged, restorers constructed a small model, consisting of blank paper artificially aged over several weeks by strong ultraviolet light and stuck on an old canvas. They covered it with some of the laminated glass that had protected the Cartoon. The police, using information from the assailant's confession, fired at the model with a sawed-off shotgun from a distance and angle that matched that from which the original was damaged.

The most difficult task, Mr. Wyld said, was to make the original flax canvas, to which the cartoon was crudely glued in the 18th century and which was distorted by the impact of the blast, shrink back to normal size. After practicing on the model, restorers used an ultrasonic humidifier to flatten the canvas. Because both the canvas and paper were fragile, the back of the Cartoon was reinforced with a layer of fine linen and buffered paper so that it now has two layers of canvas behind it. Elaborate Map Prepared

''Eric has been replacing 60-odd fragments of paper,'' Mr. Wyld said, ''some of which were one-half inch long and some that were down to an eighth of an inch. He's been gluing them one by one. He had to make an elaborate map before removing loose fragments. The fragments were photographed and each was put into its own box labeled with its location on the master plan.

''A few didn't have firm locations because they fell off when the glass hit the Cartoon. But every scrap of Cartoon we could find has been put back. The principle is that every scrap of the original is used and no restoration goes over any original Cartoon fragments.''

Mr. Harding, working with a magnifying glass and surgical instruments, is now re-establishing the texture of the drawing's surface by filling in the gaps where paper is missing with paper fiber pulp and retouching them with charcoal and white chalk, Leonardo's original materials. No attempt is being made to match Leonardo's strokes. While the artwork is undergoing this treatment, its general condition is to be examined and any necessary conservation work done.

The Cartoon was bought by the National Gallery in 1962 from the Royal Academy after one of the largest nationwide appeals ever held in Britain. It was vandalized that same year when a man threw a bottle of ink at it. The bottle did not break and no ink was spilled, but slight damage, repaired in a few hours, included scratches and a small cut.