I can answer that for you... 
This duct is for the aircon system on test in this bus,it has
appeared since last year. In the early stages of testing, there are
2 paper sheets saying " Air-Con on Test" in the interior of the
bus. Now, the sheets have been removed for a few months.I have only
seen SBS79A has this air-con test.
Result of this air-con test ( based on my observation):
- During night-time/rainy days, water droplets gathered on the
air-con itself, dripping all the way to the ground. Surrounding
seats cant be seated.
- It is quite cooling , extremely freezing at cool weathers... You
can see the window panes misty.
- Havent catch this bus lately, dont know what is the latest
performance.
How to describe the air-con in the interior? It is just a white
flat machine on top, with cold air coming out of the visible
"holes".
Ratings: Quite cold, but main problem is having dews on the flat
machine at night or cold weathers, causing water to splash all over
the rear of the bus.