Jean, I think it’s not a cap you need at this place, but a dipstick. You can find this dipstick : Vintage MG parts, To my mind the differential dipstick is available.
If you fit a dipstick to this diff in a J2, I doubt you’ll be able to get at it as it will be hidden by the prop shaft tunnel. The J2 actually doesn’t have a dipstick - there’s a filler neck on the rear cover plate and you fill it up to the bottom of the threads in the neck.