In order to be clear: this is on the front axles only.
It's basically a tube which is inserted into the axle housing.
Several holes are drilled into the axle housing, the inner
reinforcement tube is welded to the axle housing at these
locations. The kit I got had a good instruction leaflet which
I gave the workshop. Additionally, extra gussets were welded
to the axle housing ends.
Unfortunately I do not have pics ready, will try if I manage
to crawl under and take some this weekend (we have 40 deg C
and I will long think about it .......)

.