LOL! Pity I didn't know of the brush arm's importance earlier! LOL! You should have my part delivered this morning Gav.
Like I said to you Gav, I think that second hole was added by the prop makers to secure the two arms/parts together. I did a quick photoshop pic to show my theory on that second hole:
You can see how that second hole lines up with the original hole on Gavs part and mine. Maybe they added a second hole to one of the arms, so that they could secure it to the second arm using it's existing hole.
Makes sense to me but there could have been variants of that part made by Mitchell?