I just did some side by side comparisons between a magnesum 901 transmission from a 914, and an aluminum one from a 912. (I think it's technically a 902, but I believe the case is the same as a 901) The differential cover will not interchange, as the magnesium one is bigger and has an O-ring, the AL one uses a gasket. The intermediate plate will not interchange becasue one of the alignment pins is in the wrong place, and the spacing between where the bearings are and the face that meets the case is different. Also, using the aluminum transmission in a 914 would require running it upside down because flipping the ring gear would require machining a hole right through the case (there just isn't enogh room on that side of the case)
So there you have it.. they are significantly different transmissions that happen to be able to use the same gears.
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