My guess would the power supply and/or the power supply transformer. Maybe some else unique is there. You can see some thick wires through the vents in the 1st photo. What is actually there might put out some heat since the bulge has vent holes. It does seem odd to see that since China uses 220V, 50 Hz power with PAL TV so they just could have used a Euro Intellivision.
Other items of note:
The controllers are brown but the case is black with an LED (like the Super Pros, Inty III, and Tutor Pros)
The tray underneath the hand controllers has heat vents. I've never seen an Intellvision with that before.