the marine chronometer in the gimballed case is classified as HS1 with the broad arrow. HS1 is the Hydrographical Services coding for a marine Chronometer, the most accurate of maritime timepieces.
the Deck Watch or Ships Chronometer in the fixed case is classified as HS2. a high quality chronometer timepiece, but not as high a quality as the HS1. as Marcus quite rightly states, the CW means Chronometer Watch as it is technically a pocket watch in a case
the classifications goe all the way down to HS10 which is just a general issue watch for the royal navy. the fleet airarm had HS9 watches and HS8 Lemania chronographs.
its over 3 years since i actively collected marine chronometers and military watches, but most of the info is still tucked inside my brain somewhere.
if you have a UN Marine Chronometer then you are a very lucky man. they are thought to be of the highest quality, rivalling brands such as Lange & Sohne, Hamilton, Mercer etc. the number on the plaque should match the number on every piece so that you know it is original.
hope that helps a bit
Graham