The length of a window stay is measured between the pivots points when the stay is closed.

A polyfix fitting slides on/off, it detaches by putting a pin or small screwdriver in a hole near the handle.
A screw fitting has screw holes in the backet by the handle.

Adapters are available to allow screw fittings to attach a polyfix stay, these may be supplied with your order.

The 280mm stay in the picture is an auto or click clack stay
The 300mm stay in the picture is a tubular stay