Arm Rest Mounting Plate.

[B7 man]

Hello.

Im looking for advice on the base plate that secures to the floor for the arm rest.
the previous owner removed the the arm rest but the base plate is still secured in position. However the bolt that fits in it to secure the vertical mount of the arm rest is missing. Can any one confirm the size and length of the bolt that should be fitted. Also, looking for advice on how to unscrew the floor mount base plate so I can remove it to fit the correct bolt. Following picture refers to the plate I refer to and shows the bolt that is missing from mine.

Thank you in advance.