This is a unlisted embossing rare 12/A mold number described as follows:

CD: 145 Primary Embossing: (Dome) [ 12 ] (F-Crown) (Arc)W. BROOKFIELD/NEW YORK (Dome) [ A ] (R-Crown) PAT NOV 13TH 1883/ " FEB 14TH 1884 {note a line under the TH in13 and 14} Base: SB Color: Aqua

According to the CREB CD 145 research report by Chris Renaudo there have only been 3 other insulators with this mold number/letter combination reported, this is now the fourth one.

This CREB is now in Chris R's collection.