rupert: I believe you could with ease and a screwdriver put a cartridge port on the 2006 model, and maybe a TINY bit of solder and a transistor and you would be having a cartridge based 2006 SMS. Heck, I think I could do it, and that says a lot about the 2006 model. And it says quite a lot about my soldering skills. Which I have none off.
Looking at those last pictures, i wonder why they don't make it more portable. The 2006 model probably can be modded to add the cart slot (they probably had lots of boards avaiable, so they kept using it till it ended, and just removed the cart slot). Sadly the construction and wiring seems cheap to me. On early 90's, Master System and Mega Drive had great quality construction. Price is higher, but quality is lower. I wonder how that bizarre new Mega Drive looks like inside. PS: Thanks for the pics rupert!