In old cantilever levers Slr is just a spring cartridge. (It looks like a plastic coated paracetamol capsule...just in black). You can fit non slr m730 4 finger levers blades straight into m732 4 finger slr clamps. All the spring does is push against you so it feels more controlled and progressive (oh sorry mr shimano..."provides linear response"
)...and obviously gives the cantilevers extra return help as they are not pulling the lever closed.
To facilitate this, all shimano did was drill a dimple in each bit to hold the cartridge in. As soon as you undo the fixed pivot, they fly out
.
The cantilevers are slightly different as they have 2 hole positions in m732 for the spring to go into. These are marked on the spring cover slr / normal. It just winds the spring pressure up slightly. The only other thing was an upgraded cable and a upgraded brass bushing washer in an attempt to reduce friction in the system.
The suntour solution was to simply add a wound return spring. Levers are half the weight, but does exactly the same thing. But then suntours engineering was always light years ahead of shimano.
Some later adaptions included super slr, slr ev and new super slr, new super evo....the list goes on.
Your correct that some of the latter versions adopted servo wave to create a cable arc that was adjustable, but the old kit was not that complex.
