I've tried but it's beaten me, any idea how I tighten one of these up?

http://i47.tinypic.com/20qi4px.jpg

I've removed the 2 grub screws which seems to have had little effect, I've
tried lifting & sliding it forward, but don't want to use too much force &
break it.
The growler is the flush against the wall type, so no access behind. Any
pointers much appreciated.

CD