I think that the part is (Eiko #) 7506 for a single 21W bulb. There is a north american cross/equivalent but I cannot recall what it is at the moment, and my garage is too chilly tonight to venture out.
One must be careful about not using a replacement bulb that is higher in wattage and/or larger in glass bulb size. While a variety of bulbs will work, electrically speaking, if too high in wattage or if the glass comes too close to the inside of the red plastic lens, it WILL burn/melt a hole through the middle of the lens over time.
One thing you may want to consider, is getting a Beacon LED tailight assembly from Motorrad Elektrik for your R65LS - I installed one on my R65 and will be getting one for my R100 soon. The Beacon LED array replaces the internal guys of your tailight and is all solid-state LED construction, and installation is "plug and play". It will never burn out, will consume less electrical power and is brighter.