This is an ADAPTER CV joint bolt. The smooth shank is 12mm for a stock Porsche 934 CV Joint. The threads are 7/16"-20 thread.
The head and flange of the bolt have been reduced in diameter to accept a 7/16" socket wrench so the bolt doesn't dig into your axle boot causing it to tear
Available in 2-1/2" Length or 2-5/8"