Cockspur handles were the first types of window handle fitted to early double glazing, and are still common today.
The secure the window by locking across the frame - the handle ‘nose’ comes across and compresses against a ramped wedge on the frame. They are entirely surface-mounted and do not operate any sort of lock mechanism in the window.
The handles come in various ‘stack heights’. This height is the distance from the base of the handle to the underside of the cockspur ‘nose’ (see diagram)
This is extremely important to match correctly. Some handles can have a stack height of 17mm, but this size is no longer available anywhere on the market. In this case, it is best to use a handle with a 21mm stack height and fit a proportionally larger frame wedge to make up the difference.