The date when the types were separated is sometime in 1991, when the 1900D came out. The thinking is; the 300-350 are very similar, and the 1900 was only slightly different. Then the 1900D was just a little more distanced from the rest. Suddenly, they figured that it was too much of a difference from the 300 to the 1900D.
This what I was told at FSI. However, I like flit4xx's explanation better because: IT'S ALL A BIG CONSPIRACY!!!