I call these FROGs but according to Scott Kobayashi, who worked for Leon, the Frog was another project. I think Scott described the FROG as an aluminum block gun which did sound like it may have evolved into this. I did a video interview with Scott in december when I saw him at Leon's old field.
I believe they came after the Mayhem since they use Mayhem et (electronic trigger) bodies.
They are rad guns and besides the lot of them I acquired I have only seen one other besides yours. They all seem to have the same series of rental number engraved in the frame. I would say there were likely 50-100 made.
Hope that helps,