The motorcycle cops were issued new bikes just before the final scene at the docks.While Harry was investigating the vigilante murders, the SFPD got some Triumph police motorcycles to be field tested to determine their suitability. These were issued to several motorcycle patrolmen, including the vigilantes. They had just received the Triumphs when they got the call from Briggs to go to the docks. That's why they show up on the docks with Triumphs when they had been riding Moto Guzzis during the rest of the film.
WMG / Magnum Force