However, a reader who had gotten the previous 14 answers right and was upset about this question e-mailed me and claimed that he had seen a statement by Whelen himself, claiming parentage of the cartridge.
More support comes from Phil Sharpe’s Complete Guide to Handloading, which was published in 1937. Sharpe was a contemporary of Whelen’s and says flatly that Whelen was the inventor, although he doesn’t elaborate. The most recent edition of Cartridges of the World states that recently uncovered evidence indicates that Whelen was “intimately involved” with the development of the cartridge.
Most likely it was a collaboration, but in the meanwhile, until our reader can find Whelen’s statement and quote it, I will take either answer as correct.