W201 front armrest nut size

[ss201]

I'm swapping the driver's and passenger leather seat covers on my W201, as the passenger seat is like new and the driver's seat has had a lot of use. The passenger seat has the armrest fitted. I don't have a socket big enough to fit the large nut that secures the armrest to the seat frame. Anyone know what size nut this is so I can beg/borrow/buy one?

Thanks