Pentacon six MC Sonnar infinity question

[kl122002]

I am now collecting Pentacon Six and building up a collection . Just received a MC Sonnar 180 /2.8 and I checked, surprisingly I could turning the focusing ring and get passed the infinity mark behind it ? Is this normal ?
I have seen some other longer lenses , like 300mm looks like could do the same. Why ? What is that for ?

 

[P C Headland]

Mine too goes slightly passed infinity too. I never noticed it before!

 

[removedacct3]

Yep ... can confirm mine focuses past infinity as well