[Matt Monson]

Originally Posted by Jon B

According to PET, the coarse-splined 923 flanges were used in 915 transmissions up through 1979.
The transition occurred either during or after the 79 model year.

See Pete Z's timeline in post 12 here...
http://forums.pelicanparts.com/porsche-911-technical-forum/548265-83-3-0l-engine-trans-swap-85-3-2l-engine-trans-axle-options.html

In all these years, I have never once had a customer call me up and tell me I sold them the wrong LSD, and I've sold a lot of LSDs for 78-79 SCs.