When "this" and "that" are used as determiners, then the choice is based on
-- physical distance
-- emotional distance
In your examples,
(1) this stencil
The stencil is something which is nearby or will be used nearby. Therefore, you should use "this" stencil.
On the other hand, if you were distinguishing between a stencil in your hand versus the one that is on a distant table, you would compare this stencil (in hand) to that stencil (over there).
(2) that sunset
The sunset cannot be near at hand. Although you may be in the presence of the sunset, nevertheless, it must be some distance away. Therefore, you should use "that" sunset.
(3) that Extra 300S
In this sentence, the writer has already established that the Extra 300S is "one of those." The determiner "that" matches the determiner "those," therefore, you should choose "that" Extra 300S.
On the other hand, the determiner "this" matches the determiner "these."
I hope that helps! Feel free to let me know if you have other questions.