Added to my post above:
If you look closely at Siren resp. Endura and imagine where the hand will actually sist on the handle (think of the Sirens wide front guard):
On the Siren a more front part of the hand will be situated on the lockbar than it is the case on the Endura.
No idea if that influences how easy the lockbar can be disengaged accidently, but it could be that òne applies more pressure with a more front part of the hand than with a more rear part when gripping a knife- ?