This is how the Koenig Dana 18 PTO adapter looks
from behind. The adapter would bolt on at a slight angle so that
the shift lever would be more upright.
From the side you can see how relatively thin the
adapter is. A small chain inside the aluminum case takes power from
the drive gear and transfers it to the PTO shaft gear.
Here is a front view of the Koenig Dana 18 PTO
adapter. The light area is the input shaft for the adapter, the
output is on the right.