Above, Denis Payton of the Dave Clark Five on stage at the Carousel Theater, West Covina, California, 27th June 1966 – photo published in the 23rd July issue of KRLA mag. In the background an AC100 and one of the band’s new 7120 cabs. This is the earliest dateable instance of a non-Beatles 7120 cab.
Given that the DC5 summer tour began in early June 1966, it seems likely that the cabs that they had were delivered to them in late May.
Pictures taken a few months later (see below) show that the band had four.
Above, the DC5 Royal Variety performance, October 1966