A chunk of pages are missing from my copy of this Henry Altemus Rubaiyat of Omar Khayyam edition. Unfortunately, those pages probably contained more approximate information on the year of this edition. Based on web searching, similar Altemus copies of other books come from the 1890s.
Update 12/2019: I’ve discovered more about this edition with another round of searches. This Pinterest pin led me to this Etsy listing from CrookedHouseBooks—which led me to realize that the information I sought was truly here all along. I didn’t realize and understand it until I came across that CrookedHouseBooks Etsy listing that summarized it succinctly. This Rubaiyat was from a 1899-specific Vademecum binding series done by the publisher.
I’ve updated the title of the page and year metadata to conform with this new knowledge.