Just behind the door this bus's original Western Welsh fleet number 2388 can be made out, along with the new number it received in August 1974, U3064. It had been new as Rhondda 388.
Although these buses did move to other depots, despite this being at Penarth Road depot I believe it was still based at Porth at the time. It certainly was according to a late-1975 allocation list and the white diamond is also that for Porth. However, these had fallen out of use by this time and would probably not be altered if the buses changed depot. Unfortunately, the destination display doesn't give any indication of what the bus was doing in Cardiff.
From an original negative in the David Beilby collection taken by Dave Thomas