The 535 was sort of a factory attempt at including some of Fritz Egli's Bullet mods into the factory bike.
In the early to mid-90s, the Swiss motorycle modifying master Egli, of Egli Vincent fame, decided to do his magic on Bullets.
He was a pioneer for the 535cc kit, and the stroker kit with his version being 624cc, and included some various other parts like the Egli distributor and the re-designed external oil lines with the pressure-relief valve in them, and the hi-volume oil pumps.
So, Enfield used some of these Egli-type things on the Lightning engine. The Lightning has the 535cc barrel and piston, and it has the hi-volume oil pumps.
Prior to this, I am pretty sure that all the Bullets had bushings on the distributor shaft, instead of bearings. Egli made a new distributor housing, and put Swiss bearings in it, for making the ignition more precise. The old bushings would wear out quickly, and allow the distributor shaft to move laterally, which caused timing problems. Egli required that this distributor be put in as the first mod, before he would do any further tuning of the engine for any Bullet customers.
Later, the factory began to include the bearings in the distributor, and perhaps the Lightning was the first to have this mod too. I'm not sure.
I have an Egli distributor for the Bullet in my possession. I bought one of the last ones available. It's here on my shelf in brand-new condition. I have thought about making more of these, using mine as the pattern. But, since the new Bullets have the improved distributor with bearings now in the 500 models, it might not be anything but a conversation piece. But it's cool.
And here's a picture of the Egli "Super Bullet" 624cc stroker with Egli frame and bodywork.