(L'Étranger de Camus) Which part of speech is "de" in this sentence?

Question

This question is on "de" in the second sentence of the following excerpt from L'Étranger by Camus.

Ses lèvres tremblaient au-dessous d’un nez truffé de points noirs. Ses cheveux blancs assez fins laissaient passer de curieuses oreilles ballantes et mal ourlées dont la couleur rouge sang dans ce visage blafard me frappa.

Questions:

(1) Is that "de" an indefinite article or preposition?

(2) If it is an indefinite article, would it be ungrammatical in French to use "curieuses oreilles" without any article, or would that be grammatical but change the meaning or nuance of the sentence? (If the latter, what is the change?)

(3) If it is a preposition, what explains its appearance in that sentence when normally "faire + infinitive + agent" (i.e. without "de") means "let agent do" as I gather from the following examples from this Web page?

Je fais écrire David. (I'm making David write.)

Il fait manger sa sœur. (He makes his sister eat.)

Les orages font pleurer mes enfants. (Storms make my children cry.)

J'ai fait cuisiner André. (I had/made André cook.)

Answer 1

This question is related to many previous ones. Elle a de/des longs cheveux or Tout savoir sur ces combinaisons de petits mots qui contiennent « de » would be the most helpful.

1. de is the short form of the plural indefinite determiner des when followed by an adjective or an adverb modifying an adjective: de (très) belles choses
2. without a determiner, the sentence would be agrammatical
3. de is not a preposition in this case.