Yep, spot on Sean. It would be January 1979. That was the year Bay Park, Pukekohe, and Manfield hosted the three round Open Saloon Car series, with Jim Richards and John Briggs shipping their cars out to compete against the top NZ cars. Most of the local cars competing had been mothballed, as Motorsport NZ had introduced a maximum 2 litre engine limit for racing sedans following the 1977 ShellSport NZ Saloon Car Championship, due to dwindling numbers. So none of the V8s raced in the 1977/78 season, other than those racing in the South Island in OSCA, such as Inky Tulloch.

The PDL team dusted off PDL II, and added an extended front splitter at the base of the front bodywork to aid front downforce. To counter that, they also fitted that tall rear wing shown in that photo John posted, mounted on metal struts. The wing was actually off an unused McLaren F5000 they had sitting in their workshop. According to Leo Leonard, the additional rear downforce and grip from that wing was immense, allowing him to drive around the outside of Richards' Falcon sports sedan at turn one and right through the following right hand sweeper at Manfield.

Note that when the Stewart family completed the most recent restoration on this car in the mid-2000s, they fitted a tall rear wing to it, which it hadn't worn since that 1979 season. However, they decided the narrow McLaren F5000 wing didn't look too nice, so made up a new wider version instead.