Unfortunately you are not providing much information that would aid some of our historic research. Is the actual name of your grandfatherÂs canna simply C. ÂDorothy SÂ? Did your grandfather register the cultivar with the SAF (Society of American Florist) or one of the horticultural societies? What was his name?
The American Bulb Company certainly did exist in the 1940Âs but I have never seen a canna list published by them. The 1940 era was preoccupied with the war beginning in Europe and much of the horticulture industry began to decline in favor of "Victory Gardens" producing staple items for their consumption. Can you give a description for this cultivar?
Sorry that this is of little help, but there were hundreds of new canna cultivars being released in this time frame. Almost all those cannas have been lost.
Regards,
Kent