The Master-at-Arms Office was an interior room on E Deck. It wasn't really an office since it only contained the Master-at-Arms sleeping accommodation, being furnished with only bunks and a wardrobe. It served as the sleeping accommodation for Master-at-Arms Thomas King & Henry Bailey.
There are no images of the real Master-at-Arms Office, and the only film / TV to use the office is the 1997 Film Titanic. However, this film depicted the office as it was on the Olympic, with a porthole for Jack to look out and lockers with handcuff keys. Titanic's Master-at-Arms Office was an interior room, so there were no portholes.