The diamond stacks were Hollywood props (as Jeff says). The first movies films on the Silverton was in the early 1950's. If you look at photos from 1923 to 1950, you will see only straight stacks on the Mikes.
Please note that that Hollywood at the time thought that as long as a steam locomotive had a diamond stack and box headlight, the backdating to the 1880's was complete.
This is sort of like taking a Delorian or an Isuzu or a Firebird (or any other auto in the past 40-50 years) and adding running boards and a hand crank to it for it be back dated to represent a Model T Ford.
The modern era of the D&RGW NG (from 1950 to 1968) has a lot of interesting alterations to equipment. "Around the World in 80 Days" has a dead steamer (C18?) being pushed by a narrow gauge diesel dressed up as a baggage car.
Even the Bumble Bee C16 was (As I recall) was adapting the Prospector SG diesel paint scheme to a narrow gauge steamer for the Chicago Railroad Fair.
Doug vV