It's not flat? It's like what was used on at least the passenger side manifold. No gasket, bolts simply hold them together, and as long as the bolts hold, they don't leak. One other thing I noticed is that you don't have to be precise with the angle of your exhaust that mates up with the header, since it's a ball/socket, you've got a bit of a fudge factor in lining them up.
In other words, your exhaust pipe could be angled up or down, while the header outlet is essentially fixed (because the motor isn't going anywhere!). So as long as they are centered on each other, connecting the two can be easier than the flat flange style most others use. If you don't line the flat style up right, the bolts/nuts are always going to be under tension, which serves to help them work loose.