I came to this conclusion by looking at the parts in question.
The Colt fillister head screw does not overlap the locking teeth of their lock washer, therefore no locking is achieved. Also, the serrations you have seen under the head of the screw are caused by the washer being installed upside down. They are only supposed to go on one way and that is sharp side of the teeth towards the grip and smooth side towards the underside of the screw head.
This isn't a bash on Colt, this is true of most manufacturers combinations of pistol grip screws and washers.

