ahhh, sorry i was mistaken, i was thinking it was the cap. Some of them can be tricky :o)
Yeah that sounds a bit odd, most distributors have either a bracket with a bolt holding them down or some sort of clamp on the older ones i remember.
If this is a plug as you say can it be removed? See if that frees it up? It could be like a detent type plug that works like a set screw?