1929 letter to cruise ship Otranto

was bought at auction this week to add to my underpaid mail to Norway collection. The addressee was on board ship Otranto  a cruise ship belonging to the Orient line a long established British shipping company eventually absorbed into P&O in 1966. The ship was built in 1925 in Vickers Armstrong of Barrow and served in WW2 first as a troopship then took part in landing troops in the North African invasion. The first Otranto was lost in WW1 but this ship sailed on until scrapped in 1957. The address S.S. should be R.M.S. being a Royal Mail Steamer carrying overseas mail as well as passengers
The letter was underpaid therefore the addressee was required by Oslo Post to pay the purser on the ship the outstanding cost thus the unusual and rare violet cachet at the bottom of the letter.