That makes it a type III, and it is the filter screw threaded front that is rotating?
Originally Posted by jglass
It may be the front section of the lens, if the name plate ring is going around as well...
Try turning it clock wise to tighten it gently - it may be on fine threads and tighten, it which case you are ok but it will slacken the next time you remove a tight filter. If you unscrew completely cause you are curious you will admit lots of dust, it would normally be assembled in a clean room.
A repair person will address the grub screw or fix aid if you dont like.
When removing filters you need to counter hold the front of the lens rather than the rear, kitchen washing up gloves are a small investment.