1964

Good question, Greg.
I had to go back and dig out some info to find the correct answer.
In 1963, the D&RGW built 3 (of 6 total) rider gons from 9600 series pipe gons. In the same year, and by the opening of the 1964 season, they had built 6 (of 8 total) new steel passenger cars in the 330 series. These were numbered #330-335.
Since the D&RGW had not yet retired any of the remaining wood platform coaches since the loss of #280 (to Boulder in 1952), they had enough equipment with these 9 new cars to operate two trains for the 1964 season.
After the 1966 season, wood coaches #284, #306, and #320 were retired and replaced with steel coaches #336 and 337, and three additional rider gons #403-405. All were completed for the 1967 season. That was the remaining roster for the line as long as it was under the D&RGW.