From the nature of the plant, the many descriptions (here and elsewhere) invoking Wichuraiana, and the number of references cataloguing it or explicitly stating it to be a Wichuraiana hybrid, I think the "Sport of 'Red Letter Day'" parentage should be abandoned. For just two examples: Kordes 1935 has it as a Wichuraiana; 1931 Gardeners Chronicle mentions it as "a valuable and striking Hybrid Wichuraiana" (p. 38). Chaplin's indeed was producing Ramblers and Large-Flowered Climber hybrids in this era.