The Southern Pacific’s first line from Portland to California was a windy route through the Siskiyou Mountains of southern Oregon and northern California. In 1926, the railroad opened a new line across the Cascade Mountains southeast of Eugene, Oregon that was 25 miles shorter with fewer grades and curves. To serve this new route, SP inaugurated the Cascade in April, 1927.
At various times, the train was an all-Pullman affair. This included two brief months after it was streamlined in August, 1950, with two-tone-grey passenger cars (SP’s usual color scheme for overnight trains). Although SP added coaches in October, it coordinated with Union Pacific to run sleepers through to Seattle which returned to Portland on Northern Pacific’s Seattle-Portland train.
I suspect this stationery dates from the 1950s. It obviously is contemporaneous with this stationery for the Argonaut, which was discontinued in 1963.