Who would've thought that locating a metric tool to remove the fill plug on the transaxle would so difficult. According to the KB, a 3/8" female drive socket is suppose to fit and it does for a 10mm male pipe plug found on the Renault trannies. For some reason the 336 tranny on 2501, my S2, has a 11mm fill plug.
10mm four point sockets are apparently rare, 11mm four points are damn near impossible to find, at least here in the US. After searching for several days, I gave up and ordered a 7/16" eight point socket. It was a loose fit for my fill plug but by adding a shim around the plug, I got a reasonably good enough fit that I could finally remove the plug without damage.
I did find and order a 10mm square socket from Snap-On, specified for the Renault LeCar for the 10mm plugs.