I've got a pre-production/prototype M5120. Maybe uniserver can take a pic before he sends it back. My case has no markings on it whatsoever aka FCC approval or even a model number. The board is number 100 off the line and as a prototype/developers ROM chip with the word "Espirit" on it which was one of the code names of the original, M5120, Portables. This is definitely a cool find for sure, well done. Other than my board being different, the ROM chip, the prototype modem card (yellow in color rather than green), missing the label/FCC the battery cover is missing the battery icon and is smooth plastic. There is also a very unusual connector soldered onto the J21 connector on the Portable board too.

Does your case have smooth plastic anywhere at all? If so that would be a good indication that it's an early stage test/development unit.