The plates, which should have been destroyed by an explosive charge left in the vehicle, bore the suffix "HD" on the registration number, G74 OHD.
This clue to the van's origin emerged as John Major warned Sinn Fein they would remain isolated as long as the IRA continues to launch attacks like Friday night's mortaring of the biggest British Army base in Germany.
The attack on the Quebec barracks came less than two weeks after the massive bomb which severely damaged the commercial heart of Manchester and injured 206 people.
Army officials said there were no injuries in the latest blast but the attack, involving three mortars, damaged several buildings.
Mr Major, who yesterday signed a new international pact on combatting terrorism with other world leaders at the G7 summit in Lyon, said: "I think it just indicates the extent to which they [Sinn Fein/IRA] are isolating themselves, isolating themselves from the [peace] process, isolating themselves from world opinion." The terrorists were making a "fatal misjudgement" if they thought they could bomb their way back into talks.
The IRA has yet to claim responsibility, but there is little doubt in London, Belfast or Dublin that the terror group was behind it. Security sources have been warning in recent days of the danger of another attack.
The mortars were aimed at the fuel station on the base but missed. The main damage was to the Roman Catholic chapel, whose windows were blown out.