[Leica] do any of you know anything about lenses?

[leicaslacker at gmail.com (kyle cassidy on the lug)]

Hey folks ? 

I have this Canon Serenar 50 I?ve been using for 15 years as my main lens 
and recently something weird happened, the focus hits a stop about 4mm from 
reaching the infinity stop. It happened once before, I was puzzled, and then 
? it wasn?t like that anymore. 

Using my x-ray vision, I can see inside the lens and it just looks like a 
bunch of elves doing magic, but I have a vague thought that there might be a 
screw that marks the infinity point that?s come loose and sometimes sticks 
up too far.

I ordered a spanner wrench so I can open it up and look for something 
obvious, but apart from there being a loose rock inside the lens, or the 
maltise falcon, I?m not sure what to look for.

Any thoughts from the council of Wizards?

This might also be the universe telling me to buy a new lens.