Yup... it is an AEC Routemaster RM-class bus. We mostly know it as the "double decker" bus.
The last 3 digits are actually the code of the specific bus. In most cases, the code number went on as the bus's registration number. So RM731 was registered as WLT 731.
Bit of Googling revealed that RM731 was originally delivered to London Transport in 1961, made its way to Sri Lanka in 1988 and is still with Ratmalana depot, running hires for Ebert Silva.
Back in London:
... and in Colombo, with her top removed...
If you notice the just above the left fender, you