Startrite Mercury: what's this bolt for?

[Deadeye]

I've acquired a Mercury (mk I)
There's a bolt in the middle of the plate on the front (the one that details the different pulley speeds).
Anyone know what it's for? Doesn't seem to be attached to anything, and it's not to hold the plate on because there're bolts at each corner for that.

 

[samhay]

Don't think mine has that bolt. Can you post a picture?

 

[Trevanion]

The one behind the depth stop? No idea! :lol: