May be a silly question, but was there a reason behind the naming conventions for the turbo commander?
Also, why dual numbers (i.e. a 980 is also a AC695)?
In a word... marketing.
For certification purposes, the 840 wasn't different enough from the 690B to require a new model designation, so it became the 690C. However, the sales people felt that they needed to differentiate it from the 690B, so they came up with the name "840" and designated it that way. They also referred to it as a "jetprop" which was new.
On the other hand, the 980, with different engines, was sufficiently different from the 690B that the FAA did require a new model designation for it... hence "695." However, 695 is lower than 840, and we can't have the premium model number be lower than the low-end model, so it was called "980" for marketing purposes.
When the 1000 came along, it was able to be on the 695 type certificate, and thus became the 695A, and eventually 695B. However, again, "1000" was a higher number than 980, so that's what the marketing people wanted.
When the 900 came along, weirdly enough the FAA was willing to let it be on the 690 type certificate, so 690D. But, again, they needed a number that slotted in between 840 and 1000, so 900.
Does that make sense?