Just to prove that planes can be of very basic construction, I have a small user-made router.
The router is made up of 2 pieces of steel angle, each 2 7/8 in. (7.2cm) wide.
The front one is 1 in. x 1 3/8 in.
The back one is 1 3/8 in. x 1 3/8 in.
They are separated by a 1/4 in. thick piece of wood that comes out to 3/8 in to cover the top of the front angle.
The wood has a v-notch in it in which the triangular shafted blade fits. Then the 3 pieces are held together by 2 countersunk screws.
The blade has a right angle bend to then project forward and is 1/8 in. wide. The blade is made from a smoothed triangular file. A machine screw threaded through the front angle tightens the blade in position.
So there it is, small & simple. But at some point must have been just what someone needed.