The 1954/'55 rear wheel spindle in half width hubs on swingarm frames have different spacers. The drawing in the parts book show the plunger frame arrangment. For swingarm frames the two outside spacers have small flats that point towards the chain tensioners to keep them from turning while adjusting. The third spacer between brakeplate and swingarm has the same dimensions but without the flat.
I've never seen a spacer with a flat on it for the chain adjuster. I've 3 PRF wheels, two 1954 and one 1955 and for comparison one spring frame wheel which has a shorter spindle and thinner spacers as the chain adjusters bear inside the fork and not outside as on the PRF. The PRFs will fit in the spring frames with a change of brake plate but not the other way round. I've had them all for more than 50 years now and I'm fairly sure they've not been changed. Have I just been lucky or unlucky?
A picture of it loosely assembled, The size of this spacer makes that the adjuster does not touch the round part of the nut. the head of the adjuster does not bear completely on it, maybe 3quarters. Do I have a mix of bits ? or is it just a Continental thing ? both NH and KH are original fifties imports and both have the same type of nuts. Although both of them had a 'collection' of non Ariel rings and spacers on the spindle. I just assembled what I thought would be correct....but ??