I'm guessing it's a typo in your post but the M20 gearbox case number is 66-3028..not 3128...
That aside, throughout the war and up until the gearbox was changed for the tele fork models the 66-3028 number was the one used...
Quickly looking through early post war lists for different models I can't find the other number you quote...I'd avoid the case unless you can tie the number down definitively...Different numbers usually mean different parts...
I may have a spare one but will have to check...Mail me off forum if you draw a blank...Ian
That one won't do it...post 48 gearboxes were filled with oil through the front cover, whilst WD boxes were filled through a filler 'spout' cast into the main gearbox case...
As the later gear box case doesn't have the filler spout, if you fit the inner and outer cover off a WD box to the later case there isn't anywhere to put the oil in!....Ian