In my historic canna searches, I do not recall seeing a canna cultivar by the name of ‘C. Dorothy S’. If it existed, knowing where or who offered it for sale and the full name of your grandfather might be helpful. In this time frame, the Missouri Botanical Gardens as well as a number of City Parks maintained list of their horticultural plantings to some degree. If it was released to a commercial grower or possible submitted for registration we might be able to track down the source. Was their a name to go with the "S"?
There is a possibility that the name was changed by whoever submitted it for registration. Was your grandfather a gardener or horticulturalist by trade? If so, what business or organization was he associated? Did he have other canna seedlings?
As you can see, we need a great deal of information in this search; otherwise it is like the blind hog looking for an acorn.
Regards,
Kent