Schwartz' Wing 100%

Schwartz's wing, Delmore Schwartz's poetry, Ms. Schwartz's car.

Based on standard possessive rules for singular nouns ending in "z," the proper text is . Grammar Rule: Add an apostrophe followed by an ('s) to singular nouns ending in to show possession [Source 1, Source 2]. Schwartz' wing

Is this regarding a specific name, a character, or a possessive in a novel? If you tell me the context, I can confirm if "Schwartz'" (just the apostrophe) is preferred by a specific style guide (like AP Style) or if "Schwartz's" is better for your usage. Schwartz's wing, Delmore Schwartz's poetry, Ms