Diff Nut
Sooner or later, a 40+ year old gearbox is going to leak. With the Europa/Renault transaxle, it usually leaks at the axle shafts.
There is an O-ring on the shaft and a seal pressed into the differential nut. To change these seals you need a special tool to remove the aluminum diff nut.

thdif9.jpg  thdif2.jpg

Here are some pictures of my homemade tool, made from a PVC coupling for 2" pipe.
I stole this idea from the Yahoo Lotus Europa group. Click to enlarge.

thdif1.jpg  thdif3.jpg thdif5.jpg

thdif6.jpg  thdif7.jpg

thdif8.jpg   thdif4.jpg
Mark the slots with a Sharpie and file carefully. I also added a bit of 2"
pipe to increase the bite. OD is 2 3/4". ID is 2". I have not used this tool yet. But from my measurements, it should work.
Professionally made versions of this tool show up rarely on Ebay, costing from $30 to $60. This version cost me 84 cents.
My gearbox only leaks a few drops a month. But someday I will need this tool.