While the world regularly marks the anniversary of President Kennedy's assassination, usually only the closest friends and relatives of J-D Tippit recall the Dallas policeman who was gunned down by Lee Oswald less than an hour after JFK was shot in Dealey Plaza. But yesterday, the 46th anniverary of the shoootings, attention was zeroed in on a restored Dallas police car that is an exact replica of the 1963 Ford Galaxie that Tippit was driving the day he he was murdered. His widow, Marie Tippit, was escorted by her surviving children to the exact spot in Oak Cliff where Tippit parked his car when he got out to question Oswald who was walking down the street about a half-hour after he shot the President. The restored squad car will now serve as the centerpiece of a memorial to Tippit and other Dallas police officers killed while on duty.