How do I easily replicate this pattern digitally (Photoshop preffered)? It's on an endpaper from an accounting book from ~1920s.
If you have the physical paper, take a photo. The task is not trivial. The camera, lens, lights and possible cleanup edits must be pro quality. But it's still the easiest way to get a plausible result.
Why: The pattern has no periodicity in the visible area. To create this artificially one must be able to program an algorithm that fills the plane with a non-repeating pattern that resembles the result or he must have it as a photo. Finally he must find the effects for the exact look. As well one can take a photo of the real thing.
Some speculation: If one puts some grease drops (more solid than oil) between two straight and smooth planar surfaces and pulls the planes apart, he can get something that resembles your pattern. Maybe a glass and a flexible other plane to be able to get them apart easily.