Thanks to a very helpful email from Jonathan Shaheen, I just updated The Letters in the Philosophical Letters, my page that tries to say what each of the many letters is about. PL 4.23 refers, I learn, to Constantijn Huygens. As the page now says:
Topic: On assorted further questions
Reference: Includes a reference to a Mr V.Z. and his questions “concerning those glasses, one of which being held close in ones hand, and a little piece being broke of its tail, makes as great a noise as the discharging of a Gun”. Mr V.Z. here is Constantijn Huygens. One of the two letters from him to Cavendish that were published in Cavendish (1676) is on this topic (Cavendish 1676, 119-20). As published, that letter is signed “Huygens de Zulichem”; “V.Z.” would be “van Zuilichem”. Constantijn Huygens had bought the lordship of Zuilichem, and thus became the heer van Zuilichem. (Thanks to Jonathan Shaheen, who figured this reference out.)
Speaking of the Philosophical Letters, I see that Hackett are going to publish an abridged edition, edited by Deborah Boyle, in August 2021. And examination copies are free (rather than merely cheap) if you order them in April.