When the trigger is pulled all the way back and the hammer drops the cylinder should be indexed and locked so that it can't move much if at all. This is true for all revolvers. With the hammer down and the trigger all the way forward some revolvers, such as the Webley MK VI, have a part that will pop up and lock the cylinder so that it can't spin. But some other revolvers were not made this way, such as the Japanese Type 26 revolver. With the trigger forward, it's possible to accidently turn a Type 26 cylinder but not a MK VI Webley. I don't think the Spanish revolvers locked the cylinder in the rest position. However, I don't have a Spanish .455 made for British service, so maybe someone who does can inform us if it was made to lock the cylinder in the rest position or not.