It's a 570 and I'll get some more pics in the morningWhat atv is it on, year? Model? and can you take some pictures from different angles? Thanks it would help alot.
From what I can see it looks like a hex bolt to either hold the diff together, or to hold something on the inside to the case. It might not be tight becuase it could be stripped, if so you could try to put a heli-coil in it.