The OLD 2008 Silvio V1.0 fork had a square 4.6mm high chainstay engagement lip, for the chainstay, to lock to the chainstay and did not spring apart, if the skewer was partially left in the right side to allow the wheel to be removed, but stop the derailleur lug popping out due to the derailleur spring tension.
This is the Silvio S30 fork with a 45 degree engagement that is 3.0 mm thick.
This was done to reduce the overall fork+ chainstay width, to allow easier mounting on indoor trainers, and standard MTB quick releases could be used.
Due to the 45 degree lug engagement, the forks spring apart easily, and I have had to glue the derailleur lug in position, and use cable ties on the left fork, as per Ratz's link!
This is a good compromize to get disk brakes and an aero sleek frame with a 28 degree seat back!