The 1st air-con SBS service was 168, which plied from Ang Mo Kio
to Orchard Road.
However, the first SBS service to ever get an air-con bus was 143.
The bus was the Leyland National II which was a demonstrator (or
trial) bus. It lasted for a short time before the bus was sold
overseas.
Before SBS was formed, there were already air-con buses, but these
were very rare and I do not have any exact details on them.