Shimano RD-M550: Unknown adjustment screw ?

[Laowai_1]

In my continuing efforts to better understand the bits on my bike (and why they really haven't worn out after 28+ years....) could you please look at the attached photo and tell me what is the purpose of the relatively large round-headed screw with a small index mark, which is normally hidden beneath the cable clamp bolt of my rear derailleur ?

[gaz]

Spring tension adjuster.

[Laowai_1]

Got it, thanks.

[NATURAL ANKLING]

Hi,
Spring tension adjuster..................know cose I got one...............

[Brucey]

I quite liked that as a scheme; if you ran with the low setting and the shifting went bad because the cables were starting to drag, you could have a 'get out of jail free' by moving to the other setting until you fixed the cables.

cheers