This is the correct gasket for the back of the transmission.
https://www.cvi-automotive.se/en/art...30m40?q=380800
They are normally part of the M40 / M41 transmission gasket set, not the OD set. As noted, even when new they are not spectacularly effective. If you still have a leak after tightening the cover and want to address the problem, I suggest that after removing the switch cover, try to clean the mating area of oil with a fast evaporating solvent such a acetone and then use a bead of sealant such a hylomar blue on the edge of the switch cover to seal it to the back of the gearbox.