This a DIY version of the Quaife Type 9 top cover that I first saw on Guy Lowe's gearbox. It makes it much easier to check and top up the gearbox oil level. It's a simple access hole through the top of the case made with plumbing fittings. The exact parts don't matter, I couldn't get the same parts Guy used so I improvised.
The olive is not used, the nut is simply to hold the coupler in place. It's a good idea to Loctite or lockwire the nut in place.
Access is still a bit restricted on my car but a plastic knitting needle can bend enough to get into the hole and then drop under the chassis rail. We calibrated it by filling the box with 1.2L and then dipping it.
You can do this mod without taking the box out though undoing the back bolts of the cover plate is... challenging!