Its a prototype.. Its lighter than the U/L frame...
on/off buttons which will not fall out, but are removable
multiple grip mounting hole locations
even lighter than U/L frame
big trigger area
more room around circuit board area for easy wire placement
built-in snatch grip
built-in universal dovetail for direct mount on/off's
recessed mounting screws
The New Designz Proto Rail Frame is newer, so it should be commonly available, both new and used.