First off,unless it's a MXz it should have 5 bolts around the cover (check the very bottom)and then there is a bigger bolt with a lock tab,probably just above half way up in the cover which supports the shaft the reverse gear is on...snowtime said:
If it not jumping out of gear while going ahead,it could be chain adjustment,linkage adjustment,or a bad bearing....You need to undo the linkage off the arm to make it easier to get the fork out of the collar while rmoving the cover....Shavings are normal,they all make a bit...
Almost forgot,been a while,if the spacer behind the lower sprocket is installed backwards,it will allow the sprocket to work its way off the bearing and the drive pins will not stay engaged. Think it shows up in reverse first,but the fun part is when you lose drive,you also lose brakes! Make sure the bigger side of the spacer goes against the gear not bearing....
Check all your bearings while apart especially if you just lost your speedo(bad speedo drive side bearing).Sometimes it's just worn linkage not putting it all the way in...good luck