It looks like the stem/ball part may be the same. I have not measured the ball. The stem end is interchangeable: 5mm thread.
The other pieces are not the same. The bolt pattern of the foot is the same, both fit the two small holes on the cage. The big nut on the outside or the setscrew from the back are different and require a different foot design.
There is no indication of how the Lifeline mirrors actually mount. My Flickr album shows the hardware to mount the SPA or other mirrors having a 5mm female thread on the arm mount to a flat surface with 2 mounting holes that are about 38mm apart, center to center. The WESTMO does not need the big hole between the two small ones.
If you are going to bolt this type of foot on an aeroscreen instead of something flat, see
https://uberniche.co.uk/spa-mirror-shims