I suspect this enormous catchword – 5 inches of glorious end grain – was cut by Hamilton Manufacturing Co., though I have been unable to find an example of it in any of the specimens I have. The closest matches I could locate are William H. Page’s No. 12, and No. 330 by Morgans & Wilcox. However, the serifs on my block are distinctly unbracketed where the examples by Page and Morgans & Wilcox are clearly bracketed.
Many thanks to David Shields for identifying this block as Catchword No. 13 by Tubbs & Co. (see comments)