This is the standard clutch Diaphragm spring that was used in /5 and /6 models. With the introduction of the R90 S a stronger spring was used and it was retained through the 09/80 production. The stronger spring places more tension on the clutch plate, important with the more powerful motor.
The heavier spring is not needed on a stock /5 or /6 but is can be used instead if desired. The clutch lever pull will increase with the heavier spring.
If you have installed a Siebenrock Power Kit on your R75 then we do suggest that you instead use the heavier spring, part number 2121288. Also if the harder lever pull is not something that concerns you, in that case you might also rather choose the heavier spring.