That is usually the American and oriental sources cloned squarehead
screws on a Robertson screwdriver infiltrated into the Canadian market
as the same style to unknowing customers. The square socket has a sharp
ridge around the socket at the entrance end and cuts into the taper of
the driver. When styles match the locking together doesn't happen.

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"Leon" wrote in message
...

The draw back to the Robertson screw is that you can stick the screw on
the driver and drive the screw into the wood and play hell pulling the
drill and bit off of the screw. Yes this happens more often than I
wish
and normally when I am reaching into the back of a cabinet mounting
drawer slides and or supports.