Interesting. I indeed found some more info using the term M71. The Sierra diffs are not weak, but when running serious power they do fail. Perhaps this steel diff housing would be stronger, although I'm not sure in which way the diffs normally fail. I've seen pictures of the crown wheel and pinion being broken, but also of broken side and differential gears. For the side and differential gears it shouldn't matter how strong the casing is I guess.