Thanks, Jim. It must be lens distortion that makes the cartridge bases appear to be a different size. In reality, the dimensions are identical and the overall length of the mystery cartridges is 2 1/2." During the 1930's both the US and Englandicon experimented with multiball cartridges and did so again in the 1950's. I suspect that these are either experimental or very limited production rounds from Frankford Arsenal. At any rate, these appear to be an updated version of the rounds briefly discussed by Brophy.