23mm, 2pc Pacific Mail Steam Ship Company "PMSSC" Coat Button. "Scovill Mfg, Co/Waterburry" backmark.  This Company's success was anchorched in its US Mail Contracts following the Oregon Purchase, but was buoyed by the California Gold Rush. It played a critical role in moving people and goods to the gold fields and has a very interesting and colorful history (a quick internet search rewards with many historical facts).  This short paragraph is from and credited to Wikipedia:
 
During the American Civil War, the ships of the Pacific Mail, that carried the gold and silver of the western mines to the eastern states were under threat from the Confederate Navy in the form of commerce raiders, and several plots to seize one of their steamships for its precious cargo or to convert it into a raider to capture one of its other ships with such cargo. After one of these plots, that of the Salvador Pirates came to light, to prevent any further attempts to seize Pacific coast shipping, General McDowell ordered each passenger on board American merchant steamers to surrender all weapons when boarding the ship and every passenger and his baggage was searched. All officers were armed for the protection of their ships. Detachments of Union soldiers sailed with Pacific Mail steamers.[7][8]