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.
Deckplan of the Titanic, showing the real Master-at-Arms Office
Deckplan of the Olympic, showing the alternate Master-at-Arms Office
The only (wrong) portrayal of the Master-At-Arms office: a movie screenshot
3rd-Class E-Deck
Master-at-Arms Office about 1:10 A.M.