Fujinon 4/85 SF question

Hi,

does anybody own a Fujinon 85mm/4.0 Soft Focus? I bought one, and I use it on my MIN AF D7 with an M42 adaptor.

Please tell me, what the silver ring below the focussing ring is for. I can move it a bit, and it snaps back. That's it!

Thanks for any information on this lens.

Regards

Michael