Images with smoke pouring out of the smokestack were taken between 1940-1941, when the Mount was still driven by steam engines. The engines were removed for the war effort. The Mount was out of service from 1942-1945. In 1946 new diesel engines were installed and the Mount was back in service (and considerably less smokier!)
If you perform a close inspection of these images you will notice that in some of the older images there seems to be only two decks, while the later images seem to show three. This is not an optical illusion. In 1947, the pilot house was raised to the third level. Soon afterwards, additional passenger capacity was added on the third level, both amidships (which required the raising of the smokestack), and in the stern of the craft. The last major change before 1983 was to the the second level. In the stern, the second level was enclosed, while in the bow, the second level was extended all the way to the prow of the vessel.