This is very familiar. I worked on Visual and Ontel terminals with motherboard and option boards like that. The difference was sometimes only an EPROM change or a jumper clip or add to change the model from one to another.
At the last manufacturing company I worked at, they made digital proofing systems that either had 2400 dpi imaging and ran at a normal speed, or some that had 2540 dpi with a turbo speed. The difference in cost was quite substantial, but the change was only a switch flip on the motherboard and a different EPROM.
There are many other examples of this even in recent years. There are some video cards by NVidia that could go from GTX gamer video cards to their Quadro professional series cards by adding a jumper or resistor and cutting an etch on the back of the board.