So what's going on here then? Click on any picture for a hi-res image.
As we all know, Asteroids was originally release by Atari. However, in Japan, Atari licensed Sega to produce several of it's early games, including Lunar Lander, Missile Command, and Asteroids!
The board layout is almost identical to the Atari equivalent, and indeed the game plays identical to the Atari. It even states <c>1979 Atari Inc at the bottom of the screen. I've dumped the ROMs, but they are identical in every way to Atari revision 2 ROMs.
The only major difference, is a daughter board in the vector output circuit.
This plugs in where you would usually find the AD561s, and allows this board to use two "DAC IC10B"s. Exactly why Sega did this is a mystery, maybe the AD561 was in short supply in Japan at the time.