It looks like the driver is pressing the starter button on 428, ready to move from Fitzhamon Embankment back to the bus station. It is a Saturday and this may well be a duplicate bus helping to provide capacity on this busy day (particularly as the Saturday service at this time ran beyond Treorchy to Fernhill Colliery). The combined service to Treorchy and Ystrad Rhondda ran every fifteen minutes on a Saturday.
Some additional demand will come from rugby fans attending Cardiff Arms Park, visible in the background, although the ground is nowehere near capacity for the match between Cardiff and London Welsh being played that day.
The advert on the side of the bus for Arrowmint was not a common one on Rhondda buses. I remeber vending machines selling either Arrowmint or Beech-Nut chewing gum that gave you an extra packet every fourth purchase.
From a negative in the David Beilby collection from the RHG Simpson archive
