From everything presented so far then, it would appear that Fisher's intentions were that as a finished product, all of the preamps that followed the smaller chassis 400C unit were model 400CX, period. The chassis designation of the number "2" then serves as sort of a subset indicator or marker as to whether the unit itself is the rather rare early version using three 12AT7 tubes (not just one as has been mentioned), or the more common later version using four 7247 tubes. And that's where the confusion comes in.
Normally, even with significant circuit revisions within a model, Fisher did not change the model number. A good example of this is the 400 receiver. As has been well know for some time now, there were significant circuit changes from those units built up to serial number 47999, versus those built from number 48001 on -- yet the model number didn't change. It was always the model 400 regardless of serial number designation.
In the case of the 400CX however, there was one other element that changed over the course of its production, that did not change with any version of the 400 receiver -- its appearance. Early units using the 12AT7 tubes exclusively used the plated brass face plate and brown oval knobs. Later units with the solid face plate and round red knobs were always built with the CX-2 chassis. However, based on one particular unit I've serviced, the change over in appearance did not perfectly coincide with the change over from the CX stamped chassis, to the CX-2 stamped chassis. I've serviced a unit that was quite unique in that it had the brown oval knobs (i.e. earlier style appearance), yet was built with the later style chassis using four 7247 tubes -- a very rare bird in deed, although I didn't know it at the time. However, this may be the reason for the separate chassis distinction between CX and CX-2 -- simply being to keep straight which electronics an earlier style face plate unit was built with.
The bottom line I would suggest at this point then with all that we currently know, is that within the model CX line, the serial number seems to serve as an accurate indicator as to appearance, while the chassis model number appears to definitely serve as an indicator of which electronics are used. Of course, after both the appearance and electronics change over were complete, then the confusion really set in, with the finished model number having one identity, and the chassis model number having another. I appreciate all those who have commented to help in unraveling yet another Fisher mystery -- and the use of this President VI thread that first employed the 400CX in that effort!
Dave
