Do Not...
Do not use a Kirby belt on a Royal. Use Kirby belts on Kirbys only. Use Royal belts on Royals only. The wrong tension caused by the wrong belt will damage that front bearing on many Royals and of which some of those bearings are now discontinued.
Royal also makes a knurled belt for their higher amp machines but do not use them on the lower amp machine. I had been told by a Royal guru who is also a member of the vac club that machines under 5 amps should use the regular belt. The one I always use is # 1044260001. It's more pliable than what was or had always been prescribed. There is a belt with number 17398 which is used on the old Cadillac, GE, PD, and Westinghouse machines, as well as the older Royals. But, I am not comfortable using them because they are thicker and feel too hard.
If there is another Royal guru reading this perhaps you can shed some light on this subject.