Here is some of the story as I remember back in the mid 90's-Siasconset was owned/managed by the Coffin family who also owned the land that Nantucket GC was eventually built on. The land was sold to Nantucket GC and some of the adjoining Siasconset holes were overlaid by Rees Jones and his Nantucket GC design. The Siasconset holes were shortened and shifted to make the switch work.
I am not sure of when it was shorted to nine holes but it was well before we began building Nantucket GC.