The mk1 mercs had fans in the front header - air was sucked off the windscreen through vents on the canopy and then pumped through the ducting in the saloon at least that was the theory - it never really worked and the fans went at overhaul time
Interestingly enough the mk2's also had these vents in the canopy when new, although they never received the fans
Since the Perth O305s had (evaporatice) air-con, that obviously negated the need for those forced air inlets, above the windscreen. For years, I had been wondering why they had plating, over the windscreen, underneath the header:
Images as credited, with thanks to the perthbus.info archives.
Here is a good example of how they looked.
Front shot of the O305. Here, you can clearly see that the air-inlets have been plated over.
Air-con ducts with individual, Smith's air nozzles. Also visible, are those circular air-ducts. They appears as nodes on the roof in the first pic.