The two sketches were done with an altered felt tip marker.
By altered it is meant that three incisions through the wide felt tip and toward the barrel were made with a utility knife. This generated the look of the triple line brush stroke.
Take the amount of lines you see in parts of the sketches and divide by 3. Those are the number of strokes that produced them.
The technique allows single, multiple colors or even a texture to be used in back of the film.
If the negative film of the swamp had red paper in the back, because of the luster of the film, it would look like red ink.
One can easily take a Flair felt tip pen and give it a chisel point either by using a utility knife or a pair of scissors.
One can also alter the felt tips in other creative ways, even getting a spongy effect when the tip is heavily butchered.
Some chisel pointed felt tip pens are available for purchase but the methods above can customize to suit needy situations and will not be found in the art supplies section.
If nothing else we could say “I did it my way”.