I think this will be a Ford MT75 gearbox. I have a Mazda box but I think on the Ford box, there may be a bolt with a breather hole at the very top of the box. As long as the hole is clear, leave it alone and use the plug on the side of the box as both a filler and leveller.

Doug
2011 Plus 4 in Rich Maroon

1972 750 “ComDom” sprinter
1958 Triton 650
1992 Triumph Trophy 900