I dug up Merlin, and the PCB is practically identical to Master Merlin. The size is the same and all the components are in the same place. The only differences are that the traces are wider, which caused a few re-routes, and the OSC cap and resistor are different.
In fact, the date code stamped on the Master Merlin PCB is 8027; the copyright on the case is 1982, so I'm pretty sure that means the PCB was meant to go into a Merlin. Probably sales dropped off and they were left with an inventory of PCBs.
I guess that explains the genesis of the 1400CR - they wanted the extra ROM space, but needed the same pinout. I wonder if any other devices use the 1400CR.
The "buttons" are a single sheet of plastic folded in thirds with conductive paint applied to 2 of the sections. The 3rd section folds in between the other 2 and has holes to allow the other 2 sides to touch when you press on it. The Master Merlin version is also mostly identical to the Merlin version; there are only a few minor differences. The button layout, mounting holes, and connector are the same. I can't find a date code on it, but I assume it was also leftover from Merlin.