Steve McQueen 'cameo'
Let me say this first, I think it is an Steve McQueen 'cameo'.
Later on in the movie when David Toschi has a new partner in the late 1970s, there is a scene when Toschi gets a phone call that a new Zodiac letter has arrived. In this scene you can see his new partner, who looks, in my opinion, like Steve McQueen. Specially during the movie Bullit (1968).
http://oi67.tinypic.com/2me5z74.jpg
This matches with the fact "Steve McQueen copied Toschi's distinctive style of quick-draw shoulder-holster by wearing his gun upside down for the 1968 movie Bullitt. McQueen also modeled much of his Bullitt character on Toschi".